To provide a start assisting device which is compact, lightweight, and easy in mounting, and to increase the loading capacity through the weight reduction by directly changing the load distribution of a driving shaft and a driven shaft by a hydraulic cylinder.

A clevis (U-link) 14a is connected to a side face part of one end of a spring pad 14 to constitute a trunnion type suspension in an approximately orthogonal manner relative to the spring pad 14. The clevis 14a is connected to a rod end 20a of a hydraulic cylinder 20 at its tip part through a clevis pin 19, and a cylinder end 20b of the hydraulic cylinder 20 is turnably fixed to the side face part of a frame (side frame) 11. The hydraulic cylinder 20 is automatically driven and controlled by switching a hydraulic circuit by a control unit based on various conditions such as loaded condition of the vehicle.