PURPOSE: To prevent a compressor from expanding and compressing air in its crankcase, by respectively reciprocating a pair of pistons through a pair of connecting rods connected with a single crank cam.

CONSTITUTION: If a prime mover 28 rotates a rotary shaft 26 to be driven, a compressor, reciprocating while swiveling a pair of connecting rods 33, 33 connected with a crank cam 27, reciprocates pistons 37, 37, connected with said connecting rods 33, 33, to be slided along each cylinder 21, 21. Here the moving direction of each piston 37, 37 is continually equal. That is, a distance between each piston 37, 37 is always held to a fixed value. Accordingly, the compressor, causing no change of the displacement in a crankcase 22 positioned between the pistons 37, 37, prevents fluid in the inside from being expanded and compressed.