To install a prefabricated building unit in a narrow lot with ease and to secure both of a space as a parking lot and a room as an office or a warehouse.

An prefabricated building unit is provided with a lower floor frame part 3, in which a lower space part 2 for a parking lot is arranged inside, and an upper room structure part 7 which is placed on the lower floor frame part 3. Front columns 13a of the lower floor frame part 3 are arranged to retreat more than the front edge of a floor wall part of the upper room structure part 7. The lower frame part 3 is provided with a frame 8 placed on the ground, which is formed into a U-shape opening forward in a plan view, in the lower edge.