To provide a power supply apparatus which consumes little air, has high reliability, and has high conversion efficiency, and to provide a system employing the apparatus.

This power supply apparatus comprises a pneumatic actuator unit 11, composed of a cylinder 13 and a piston 14 and a power generating unit 12, which converts the linear motion of the piston 14 into a relative motion between a coil 38 and a magnetic circuit 44. The coil 38 is attached to the tip of the piston 14 via a bobbin 41 and the magnetic circuit 44 comprises a yoke 37, a pole piece 39 and a magnet 40. The piston 14 is moved straight reciprocated in a straight line by supplying of compressed air.