K-pop is popular all over the world, but Japan is really the place where K-pop idols have to succeed if they want to be successful. With the second largest music industry in the world after the United States, Japan is a goldmine of income for K-pop stars, especially K-pop acts that go unnoticed in Korea's relatively small but oversaturated music industry. Many of the most popular Korean acts are popular in Japan, but there are even some that have varying popularity Korea that have done well in Japan.

 The Boss

The Boss has had a lot of ups and downs. All five members debuted in rotating group Xing, then became to the Boss in March 2010 and promoted two songs. But  by March 2011, the group released its first Japanese song and ended up chart in sixth place on the Oricon Daily Chart. Since then, The Boss primarily promoted on Japanese shows, with occasional ventures to Korea. The group has released several Korean albums, but in 2012 the CEO of The Boss's company, Open World Entertainment, was arrested right before The Boss released its first official album and the album was cancelled. Because they were still signed to a Japanese agency, The Boss continued to promote in Japan.

The Boss has released twelve different albums in Japan, including two video albums and a live album. The Boss's vocal-pop appeals to Japan's audience, and most of the group's albums have charted on Oricon in the top ten. The Boss has yet to release a full Korean studio album, but has released two EPs and four singles, including the latest "Rilla Go!" in 2014. The group admitted that "Rilla Go!" is a change of style from what The Boss has released previously in a last attempt to gain attention in Korea.

SHU-I

Another group that initially debuted in Korea but after being unable to make a huge dent in the K-pop industry SHU-I went to Japan, where they proved to be more successful. SHU-I's so Japanese focused that the group no longer has a Korean website, and their YouTube page primarily posts videos with the members speaking Japanese.

SHU-I has released eleven albums, but only one Korean mini-album and one single. The group recently released a seventh single, "Mirai E" in Japan on December 3.

MYNAME

And yet a third group that went to Japan after debuting in Korea. MYNAME did pretty well, and was produced by Fly to the Sky member Hwanhee. Two of MYNAME's singles appeared in the top ten on Korea's Gaon chart, but the group proved to be much more popular in Japan. They keep flitting back and forth between Korea and Japan; MYNAME's latest album released was "Five Stars" in Japan.

Kara

The only girl group on this list is possibly the most popular Korean artist in Japan. Kara's very popular in Korea, but its popularity in Japan is almost absurd. Kara was the first Korean girl group to hold a concert at the Tokyo Dome, and has released seventeen Japanese albums and singles in comparison to the eleven that the girl group has released in Korea. Several of Kara's Japanese albums have gone platinum, two have gone double platinum, and "Super Girl" went triple platinum in 2012.

 Kara's often compared to Girls' Generation when it comes to talking about Japanese success, but Kara's not been one of the top girl groups in Korea since the group spent much of its time in Japan. This changed in 2014, when Kara made a comeback with "Mamma Mia," and the newest member Youngji started gaining popularity on variety shows. Hara also recently gained her own variety show, signifying that the girl group is making a greater attempt to stay in Korea.

Apeace

Debuting previously as Double B 21 in 2010, Apeace started with 21 members (and went by A-Peace until changing the stylizing) and now has fifteen members, who are split into subgroups: Lapis5, Jade5, and Onyx5. Despite being made up entirely of Korean members, Apeace hasn't released anything in Korea since the 2011 mini-album "We Are The One" prior to changing its name from Double B 21. After the name change, Apeace promoted solely in Japan. Even though Apeace has only promoted in Japan, there have been several Korean songs on their albums. The group's song "Veil" appeared on Billboard's Japan Hot 100 chart in 2014, the first time that the group appeared on that chat.   

Apeace has picked up a Japanese trend, and fans can see shows six days a week in a theater in Shibuya, Tokyo.

TVXQ

Honorable mention to TVXQ. The group has always been popular, but TVXQ has actually released more albums in Japan than in Korea. The group recently released "With" in Japan.