PURPOSE: To lower the center of gravity of a machine body by miniaturizing a transmission case of a self-travelling type working vehicle.

CONSTITUTION: An engine E turning a crank shaft 34 downward is supported by an engine bed 2 protruded forwardly from a transmission case 1. The crank shaft 34 is connected to an input shaft 35 of a transmission 61 by a belt tension clutch 36 arranged in a horizontal face and simultaneously the output shaft 67 of the transmission 61 is connected to the wheel shaft 101 by gears 88 and 106. By the above-mentioned layout, the position of the belt tension clutch 36 can be lowered without interfering with wheels W and thereby, miniaturization of the transmission case 1 in size in vertical direction is made possible.