To improve mounting performance to a vehicle and responsiveness and accuracy in adjusting a vehicle height as to a shock absorber having a vehicle height adjusting function, and to reduce consumption energy and confirm performance without having the shock absorber mounted on the vehicle.

For attaining the purpose, this shock absorber has a cylinder 1, a piston 2 slidingly inserted into the cylinder 1, a piston rod 3 movably inserted into the cylinder 1 and having one end connected to the piston 2, rod-side and anti-rod side two operation chambers R1 and R2 partitioned in the cylinder 1 by the piston 2, a passage 4 making the respective operation chambers R1 and R2 with each other, a gas spring S for making pressure act in the respective operation chambers R1 and R2, a pressure adjusting piston 5 slidingly inserted into the cylinder 1 and an actuator A for driving the pressure adjusting piston 5. The vehicle height is adjusted by adjusting volumes in the respective operation chambers R1 and R2 by driving the pressure adjusting piston 5.