PURPOSE: To make a floor high or low in height, and thereby lower cost by constituting each mount base on the upper side of which floor frames supporting a floor plate, to be changed in mounting height with each mount base vertically turned over with respect to a body.

CONSTITUTION: When the high mounting of a floor plate 1 is made to be a standard one, in this case, the support position of each mount base 3 on which floor frames 2 are mounted, is in the high level where the floor plate 1 is in a high position with respect to a body 4. When the low floor configuration is to be employed, the support position of each floor member 2 can be lowered by mounting each mount base 3 onto the body with each mount base vertically turned over from the mounting condition of a high floor configuration, so that the support position of each floor frame 2 can thereby be lowered. Thereby this permits each floor frame 2 and the floor plate 1 and the like which are mounted onto each mount base 3, to be lowered as compared with the standard height, so that the floor can thereby be made high or low in height with each mount base 3 vertically turned over. By this constitution, assembly works can be made simple, and thereby making it possible to lower production cost because each mount base 3, each floor frame 2 and the like can be commonly used.