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. A frusto-conical projection is formed on the top of the piston at a point offset from the axis of the piston so as to be receivable in the discharge port which also is offset from the axis of the piston, thus minimizing dead volume when the piston has reached its top dead center of its stroke.