PURPOSE: To enable a cooler unit to be mounted or dismounted without difficulty while securing the floor area for storing, by allowing all or part of a wall surface to be retracted into a box.

CONSTITUTION: When it is required to mount a cooler unit 11, a screw 41 securing rear and lower wall surfaces 32, 29a is unfastened and the wall surfaces 32, 29a are swung inside of box main body section 27. Thus, the rear wall surface 32 can be recessed from the locus of a line along which the cooler unit 11 is moved to be attached to secure the attaching space. In this state, after the cooler unit 1 is attached and flanges 17, 17 are secured, the tentatively attached wall surface 32 is swung to the original position, and the lower wall surface 29a is secured by the screw 41 to complete the attaching.