The method includes in succession a first step of assembling a power unit into a sub-frame in almost the same position as in a mounted state on a car body frame by interposing a torque rod and at least two on-board mounts between the power unit and the sub-frame; a second step of attaching the sub-frame on which the power unit is mounted to the car body frame; and a third step of supporting the power unit via a power source side mount and a speed reducer side mount on the car body frame.