a housing having a front housing and a rear housing and forming a plurality of first cylinder bores, a plurality of second cylinder bores and a suction chamber formed in the rear housing, the rear housing being located rearward of the second cylinder bores;

a rotary shaft rotatably supported by the housing and having a rotational axis, the rotary shaft having an inner chamber along the rotational axis, a first suction communication passage and a second suction communication passage, the inner chamber communicating with the suction chamber near a front end of the rear housing, wherein the first cylinder bores and the second cylinder bores are arranged around the rotational axis of the rotary shaft;

a plurality of double-headed pistons connected to the rotary shaft, each of the pistons being accommodated in the first cylinder bore and the associated second cylinder bore to respectively define a first compression chamber and a second compression chamber, each of the pistons reciprocating for compressing gas in the first compression chambers and the second compression chambers as the rotary shaft rotates;

a partition wall located in the inner chamber along the rotational axis of the rotary shaft for dividing the inner chamber into a first passage and a second passage, the first passage interconnecting the suction chamber and the first suction communication passage, the second passage interconnecting the suction chamber and the second suction communication passage, wherein the partition wall has a rear end portion that is closer to the suction chamber than a front end of the second communication passage; the gas in the first passage and the second passage maintaining substantially the same pressure as in the suction chamber, wherein the front end portion of the partition wall is fixed to an inner circumferential surface of the inner chamber so that a front end of the first passage is located frontward of a front end of the second passage and so that the first passage and the second passage are separately defined from each other;

a first suction valve mechanism rotatably provided on the rotary shaft near a rear end of the front housing for introducing the gas from the suction chamber to the first compression chambers through the first passage, the first suction valve mechanism including a first rotary valve that has the first suction communication passage for sequentially interconnecting the first passage and the first compression chambers in a suction process as the first suction valve mechanism rotates synchronously with the rotary shaft; and

a second suction valve mechanism rotatably provided on the rotary shaft near the front end of the rear housing for introducing the gas from the suction chamber to the second compression chambers through the second passage, the second valve mechanism including a second rotary valve that has the second suction communication passage for sequentially interconnecting the second passage and the second compression chambers in the suction process as the second suction valve mechanism rotates synchronously with the rotary shaft.