Abstract: There is provided a fixed displacement suction and exhaust apparatus using coaxial rotary pistons, which can be used for various kinds of compressors, vacuum pumps, fluid transfer pumps or internal combustion engines. In the suction and exhaust apparatus, two rotary pistons 16a and 16b having a plurality of wing plates 17, 57, 18 and 58 radially provided in a housing 2 having a cylinder and two sealing plates, are engaged with each other to be rotated. The rotary pistons are rotatably coupled to two connecting rod elements 41a and 41b of a connecting rod 40 coupled to rotate the crank pin 15a of a crankshaft installed coaxially therewith. The suction and exhaust chambers are produced between the spaces of each of the wing plates of the respective rotary pistons as the rotary pistons rotates at different speeds, according to the teeth proportion of external gears 42a and 42b formed in the connecting rod 40 to internal gears 22a and 22b mounted on the housing.