Abstract: An electrically driven hermetic compressor comprises a hermetic casing, an electric motor unit encased in the casing, and a compression mechanism encased by the casing and drivingly connected to the electric motor unit through a crankshaft. The compression mechanism includes a cylinder, a piston reciprocatingly slidable in the cylinder, a valve plate having formed therein a suction port and a discharge port and providing valve seats around the ports, suction valve and a discharge valve cooperating with the suction and discharge ports. The cylinder, the piston and the valve plate cooperate to define a compression chamber. The compressor further has a passage system providing separate passages for a gas to be compressed and the gas after compression, and a motion converting mechanism for converting rotary motion of the crankshaft into linear reciprocating motion of the piston.