A fluid machine according to the present invention has an expansion-and-compressor device selectively operating as an expansion device for collecting waste heat from an internal combustion engine and converting the collected heat energy into mechanical rotational force, and as a compressor device for compressing refrigerant for a refrigerating cycle for air conditioner. The fluid machine further comprises an electric rotating device selectively operating as an electric power generator and as an electric motor. A power transmission device is further provided between the expansion-and-compressor device and the electric rotating device for selectively transmitting the rotational force from the expansion-and-compressor device to the electric rotating device and vice versa. The power transmission device is composed of a planetary gear train so that it can change rotational speed to be transmitted to the expansion-and-compressor device or to the electric rotating device. Accordingly, the expansion-and-compressor device as well as the electric rotating device can be operated at their most efficient rotational speed.