To improve mounting capability of another part onto the top surface of a side member without increasing the number and manufacturing manhour of parts, and restrain a transmission completely from being displaced by bounding or rebounding of a suspension.

A pair of side members 12, 12 extending in the vehicle traveling direction of a chassis frame 11 involve external walls 12a, 12a and upper flanges 12b, 12b fitted side by side on the top ends of the exterior walls 12a, 12a. A pair of mount brackets 14, 14 are fixed on the inner surfaces of the exterior walls 12a, 12a of a pair of side members so as to face each other downwards at prescribed intervals from the upper flanges 12b, 12b. A transmission 13 is fitted between a pair of side members. Both ends of a leaf spring 21 whose central lower surface is formed with the transmission are loosely-inserted between the upper flange and the mount bracket to be mounted onto a pair of side members.