To facilitate attaching/detaching work for a single engine or a transmission in an on-vehicle state by causing the constituting parts of a supporting device to have a function as a stiffener for connecting an engine side and a transmission case with each other.

An upper plate 7a is joined and fixed by an attaching bolt to the lower part of almost the centroid of a power unit, a lower plate 7b is joined and fixed by an attaching bolt to the cross member of a car body and thereby the weight load of the power unit in a vertical direction is elastically supported by a mount rubber 7c. By fixing the upper plate 7a of this supporting device 7 over an engine side and a transmission side, the supporting device 7 is caused to have a function as a stiffener for strengthening the lower portion of a connection part between the engine side rear part and the transmission side front part. Thus, attaching/detaching work for a single engine or transmission in an on-vehicle state is facilitated.