After participating in the later half of the series Taiyō ni Hoero!, he moved to the art department to work on props and decorations, and participated in many films, including Gamera: Guardian of the Universe and Gamera 2: Attack of Legion.
Hasegawa began writing for television in the 1990s and has been a writer for almost every Ultraman series produced since Tiga up to the first season of Ginga.
Hasegawa has been lauded in the field of tokusatsu writing for his dramatic shifts in tone regarding the content of scripts.
A 2004 issue of Hyper Hobby has credited him as the creator of the Tokusatsu Drama craze in where the narratives are portrayed in a style similar to J-Drama rather than traditional tokusatsu stories, which were called by the editor of the magazine as "shallow and unproductive" by the early 90s.
In many interviews, Hasegawa has stated that he feels tokusatsu heroes should be somewhat tormented and have to rise above adversity.