An expansion-and-compressor device of a fluid machine has a pump mode operation in which the expansion-and-compressor device is rotated by a driving force from a driving source to compress working fluid, and a motor mode operation in which the fluid pressure is converted into energy of movement to output kinetic energy. The fluid machine further has a transmission device for transmitting the driving force from the driving source with increased rotational speed, when the expansion-and-compressor device is operated in the pump mode operation.