Following his graduation from Keio University High School, Hiroshi moved to Keio University's Faculty of Arts and Literature, and eventually made his professional debut writing the script for the 16th episode "Kung Fu Dandy" of Super Dimension Fortress Macross. He also wrote the light novel Misa Hayase: White Reminiscences. Haruhiko Mikimoto, Shōji Kawamori and manga artist Fujihiko Hosono are also alumni from his high school.
Hiroshi is known mostly for writing scripts for science fiction robot animation such as Heavy Metal L-Gaim, Super Dimension Century Orguss, Genesis of Aquarion and Mobile Suit Gundam SEED, but he has also worked on mundane comedy series such as Atashin Chi and Doraemon. He has also written scripts and organized series composition for Earth Girl Arjuna and Fullmetal Alchemist FULLMETAL ALCHEMIST.
He also worked on the novelizations for series such as RahXephon and Royal Space Force: The Wings of Honnêamise.