To attach a staircase unit efficiently to a floating body structure by forming it into a more self-standing type by means of a boxy structural body consisting of a steel frame, a ceiling panel spread over this frame, a wall panel and a floor panel, and a stair beam built in this boxy structural body and a handrail member.

A boxy structural body consists of a steel frame 1 construct into a boxy form, a ceiling panel 2a made up of spreading a panel material over this frame 1 and a wall panel 2b as well as a fllor panel 9, and a handrail member 3 and a stair beam 4 are built in this structural body. A door opening 10 is properly incorporated in the wall panel 2b of a self-standing type staircase unit 5, and a cable trunk is formed in the steel frame 1 at a ridge part of the boxy structural body. With this constitution, the stair beam 4 and the handrail member 3 are built in the boxy structural body and then this self-standing type staircase unit 5 can be efficiently manufactured.