A plurality of pistons are provided in the associated cylinder bores arranged around the rotary shaft. Each piston reciprocates in relation to the rotation of the rotary shaft to execute suction, compression and discharge of the refrigerant gas. A valve chamber is disposed in the vicinity of the cylinder bore. A plurality of ports are arranged to communicate the valve chamber to the associated cylinder bores. The valve chamber has a rotary valve which rotates in relation to the reciprocating motion of the pistons. A suction passage is formed in the rotary valve to be sequentially communicated with each port in synchronism with the rotation of the rotary valve in order to introduce the refrigerant gas into each cylinder bore. A seal zone is included in the rotary valve, which sequentially blocks each port in synchronism with the rotation of the rotary valve. A groove is defined in the rotary valve, which is capable of guiding the refrigerant gas entering between the valve chamber and the rotary valve.