To reduce the cost of casting by remarkably allowing free layouting and increasing a workability for assembly and piping.

This Roots fluid machinery comprise drive side and driven side rotors, a rotor chamber for storing both rotors, a casing having an inflow passage 19 for fluid communicating with the rotor chamber and the inlet thereof and an outflow passage 21 and the outlet thereof, an input pulley 7 disposed at the axial one side of the casing 9, and a timing gear set for rotating both rotors in synchronism with each other. The inlet and outlet are provided on the axially opposite side of the input pulley 7.