A compressor having a casing to which a gas suction pipe is connected; a driving motor provided in the casing; a cylinder; a valve supporting plate (210) covering the cylinder (300), the valve supporting plate (210) having a suction hole (212) for sucking gas into the cylinder (300) and two discharge holes (211) for discharging gas compressed in the cylinder (300); a piston (100) having two protrusions (101) at a pressure surface in correspondence to the two discharge holes (211) of the valve supporting plate (210), the protrusions (101) having different sized cross-sections, the piston being linearly reciprocal in the cylinder (300) by receiving a driving force of the driving motor; a suction valve coupled to the valve supporting plate (210) to open and close the suction hole (212); a discharge valve coupled to the valve supporting plate to open and close the two discharge holes (211).