To exert a vibration restraining effect to a supported mass even when a natural frequency of a vibrating system constituted by a vehicle body supporting device and the supported mass supported by it is changed.

In this vehicle body supporting device 1, a cylinder 2 is split into a first air chamber 4A and a second air chamber 4B by a piston 3 moving forward and backward in the cylinder 2. The first and second air chambers 4A, 4B are filled with gas. The piston 3 relatively moves forward and backward to the first and second air chambers 4A, 4B to input the vibrations of a vehicle body 100B in the first and second air chambers 4A, 4B. The first and second air chambers 4A, 4B are connected with each other through a gas passage 7, and a gas passage opening/closing means 8 provided to the gas passage 7 is opened/closed at a predetermined frequency.