To provide a damping force changing device for a shock absorber with a simple mechanism at a low cost.

A shock absorber device is constituted of a shock absorber main body l damping vibrations and rubber bushes 7, 9 provided at fitting sections when the shock absorber main body 1 is fitted to an object to be fitted. A sealed circular space 7b is formed in one rubber bush 7, and it is connected to an air reservoir tank 12 via a pipe 11 provided with a solenoid valve 10. The solenoid valve 10 is driven and controlled by a control unit 13, the circular space 7b is opened to the atmospheric air when it is not driven (off), and air is fed to the circular space 7b from the air reservoir tank 12 when it is driven (on). The spring constant of the rubber bush 7 is changed by the change of the internal pressure of the circular space 7b, and the damping force of the shock absorber device can be varied.