TLDR: Idol Song is essentially J-Pop sung by Idols instead of J-Pop singers. There are no musical requirements for a song to be classified as Idol Song, but there are common conventions that one can expect when they listen to music sung by an Idol.

Idol Song (アイドルソング) is the name of one of the pseudo-genres from Japan surrounding Nerd Culture. Idol Song is the name of songs sung by idols. The most popular comparison is something called Anisong (アニソン). Anisong is any notable song that appears in Anime (mostly Opening Songs and Ending Songs). Another psudo-genre is Game Music (ゲームミュージック), Game Music is notable music that appears in Video games. Denpa Song (電波ソング) is a type of music popular in Akihabara (The physical hub of Japanese Nerd Culture in Tokyo).

I call these genres pseudo-genres“ because unlike a normal genre that have uniting musical conventions like BPM ranges, instrumentation, song forms, common chord progressions, etc., these pseudo-genres have no uniting conventions. They become the pseudo-genre merely by appearing in the type of media. Game Music is Game Music if I was placed in a Video Game, period. It doesn't have to be chip-tunes, nor classical, nor looping, nor rap, not catchy, not midi, not live. If a developer put a song in a video game it becomes Game Music, period. Likewise, no matter the musical genre, the lyrical content, or the instrumentation, Idol Song becomes Idol Song if a person identifying as an Idol sings the song. This is not to say, as we will discuss later, that there are not cultural expectations around what Idol Song is, that can either be played straight or subverted.

This song is a PERFECT example of Vanilla Idol Song.

Momoiro Clover Z - Z Densetsu

Note that the music is high energy, apolitical, there is a dance routine, the outfits are color coded, there are 5 girls in the group, and the girls are singing in cute voices.

In general, Idol Song is just J-Pop sung by Idols. So to understand Idol Song is to understand J-Pop. The more you study J-Pop, the easier it is to recognize the cultural expectations of Idol Song that likely will be present in any given song you hear by your favorite Idol. Most Japanese Nerd genres are built on top of J-Pop, so understanding J-Pop, again, will help you understand Game Music or Anisong. So most everything I talk about will be in reference to J-Pop. The things I talk about in this entry is Vanilla Idol Song, there are many flavors (ie. electronic music idol sounds, denpa idol sounds, disco idol sounds, etc…), but I'm just talking about the Vanilla versions.

I have no idea where, but I once heard someone say that the Idol Industry is “A little Embarrassing.” Its embarrassing for the Idols, wearing those costumes and singing crazy songs to middle aged men. And its also a little embarrassing for a middle aged man to support this Idol he has no chance with by giving her enough money to start a college fund… but I digress.

Idol Song is normally: sung by a group (3+ members) of cute young women; has amateur level vocalists; involves lyrics concerning puppy love; involves sugary sweet lyrics; involves the vocalist singing in high pitched cute voices; involves lyrics concerning unrequited love, having fun, or other youthful centered activities ; cute instrumentation; upbeat (over 140 BPM); musically unsophisticated (4/4 time, simple harmonization, little to no key changes, etc…); and sung in unison or with simple voicing. If you hear a song that checks more than 3 of these boxes it is almost certainly Idol Song.

Further more there are some melody specific nuances that have become pretty common place in Idol Song. The melody will frequently leave empty bars or sections so that fans can cheer in the empty spots. Often times it will be something like a bar of melody and a bar of silence or 14 beats of melody and 2 beats of silence. Non-ballad Idol Song melodies are specifically crafted with fanchanting in mind. So another dead give away is whether you could chant a 2 or 3 syllable phrase after the Idol stops singing a line. But this is not exclusive to Idol Song as Enka's melodies are written the same way (but fans may still be chanting though, even in Enka).

Look at this video with Enka Music. The melody could be chanted to if it was faster. Also note that the singer is a real songstress and very vocal ability is at a professional level. The lyrics match the tone of the song and are sad. The singer is a cute young woman, and the singer did not write or compose the song, but notice her skill in singing. The singing level is a dead-give away that its likely not Idol Song.

津軽海峡･冬景色　石川さゆり

Here is a link to the lyrics.

It should be noted that Idols are not a monolith and every Idol Group sounds different. But all Idol Groups will have a vanilla Idol Song in their song list.

I provided another example of a completely vanilla Idol Song below. I can point out many things to consider in this song. It has the vanilla song structure (Intro, A melody, b Melody, Sabi, repeat, then instrument solo, and 2 chorus, then finish with an outro), many of the conventions, and it has a great example of fanchanting (it has both the call and response parts of the fanchant clearly audible). The structure is textbook, it has an instrumental solo and a “Falling Sabi” (a quiet 3rd Chorus) and an outro. The Intro is an open 8 that is used for cheering. The lyrics concern young puppy love. Its upbeat. This is quintessential Idol Song.

Ni on Ashi Dancing (Up Up Girls 2)

Bubblegum KK (Animal Crossing)

This song ticks a lot of idol boxes as its a “parody” of the genre. Note the melody, not the “feel change”, note the instrumentation.

Fan Club (Rhythm Heaven)

Note the instrumentation and the melody.

Some Oricon Top hits in 2020