Shimomoto was born in Osaka, Japan on August 14, 1948. He started his career as an actor in director Banmei Takahashi’s pink films in the early 1970s. He has worked with Takahashi in many other films and has also starred in the works of other noted filmmakers, including, Kōji Wakamatsu, Akira Fumakachi, Kazuyuki Izutsu, and Shūji Kataoka among others, amassing a filmography of more than 250 pink films. He was given a Best Actor award at the first annual Pink Grand Prix covering the year 1988, and a second in 2006 for his performance in Shinji Imaoka's Mighty Extreme Woman aka Uncle's Paradise. Shimomoto wrote, directed, starred in and did the cinematography for 離婚妻快感ＯＮＡＮＩＥ, released by Shintōhō in 1992. Shimomoto was given a Best New Director, 2nd place award at the Pink Grand Prix for that year. Recently he has also starred in mainstream films and TV shows.
^Filmography based on Weisser, Thomas; Yuko Mihara Weisser (1998). Japanese Cinema Encyclopedia: The Sex Films. Miami: Vital Books : Asian Cult Cinema Publications. ISBN1-889288-52-7.and"下元史朗" (in Japanese). Japanese Movie Database. Retrieved 2009-03-12.