A multi-cylinder piston-operated compressor having a combined cylinder block closed on both axial ends by front and rear housing and provided therein with a reciprocative piston mechanism, for sucking, compressing, and discharging a refrigerant gas, and a connecting flange, from which the refrigerant gas after compression is sent toward a cooling circuit. The connecting flange defines a muffling chamber in cooperation with a circumferential section of the combined cylinder block for suppressing the pulsation in discharge pressure of the refrigerant gas. The compressor further has a gas expansion chamber in which an expansion and a partial collision of the compressed refrigerant gas spouted from orifices take plate to weaken the pulsation in the discharge pressure of the gas with the least discharge loss.