To surely support a rolling axis being the rolling center of an auxiliary driving system, and enable the translational motion of the auxiliary driving system to be suppressed by constituting a front mount so that the intersection of the elastic principal axes of the front mount is positioned on the rolling axis that passes through the center of gravity of the auxiliary driving system.

An auxiliary driving system 20 that is to be provided in a midship type power plant mechanism in which an engine 30 is arranged in the lower position of the floor 34 is provided with auxiliary machines such as a compressor for an air conditioner, alternator, power steering pump, and fan 21 so as to be unified. In this case, a front mount 23 (23a, 23b) is supported on a suspension 25, and rear mount 24 is supported on the floor 34, thereby the auxiliary driving system 20 is supported in a vibration control condition. Denoting by A the rolling axis passing through the center of gravity G of the auxiliary driving system 20, the elastic principal axes FMa, FMb of the front mount 23 are made to have an intersection with the rolling axis A.