Kongai Renai 2002 (Japan)

Kongai Renai 2002 (Japan), also known as Love Outside Marriage, Extra Marital Love, Kon'gai Ren'ai is Japan drama premiere on Jan 10, 2002 on TV Asahi

Plot Synopsis by DramaWiki Staff ©

Can a married couple maintain their relationship while each of them has their own and "official" girlfriend and boyfriend? A bold and interesting test begins!
The wife Mitsuru and the husband Takuya is a three-year married couple who enjoyed a rather "friendly" relationship. Although Mitsuru is always busier than Takuya, both of them had no complaints. But ever since they met another couple, Shinichiro and Shizuka, their relationship rapidly became unstable. This is because Shinichiro asked Takuya to date with his wife Shizuka from time to time, which of course Takuya refused at first. However, his wife Mitsuru agreed as she considered this proposal will not cause any negative effect on their relationship as long as they were just having a casual date. This drama will lay out an eternal theme between a married couple, "Is it hard to maintain passion and affection toward your wife/husband after marriage?"
(Source: Wiki.d-addicts )