Disclosed is a process for producing a thiabenzoazulene-propionic acid derivative which is useful as an active ingredient of an antihistaminic agent or the like. According to the producing process of the present invention, it is possible to produce a thiabenzoazulene-propionic acid derivative where the 2-position of the thiabenzoazulene skeleton is substituted with propionic acid. The thiabenzoazulene propionic-acid derivative thus synthesized has excellent antagonistic action to histamine H1 receptor and low intracerebral transmigration and, therefore, is useful as an active ingredient of the pharmaceutical composition such as an antihistaminic agent.