To prevent confinement in a roots type fluid machine to reduce noises emitted therefrom by forming run-off parts at the dedendums of respective rotors, which define space between them and the addendums of the mating rotors respectively for preventing the undercut of the dedendums, and forming notched parts at the tooth traces of the rotors on at least its one side in the rotational direction for making the space open to the rotor casing.

Rotors 15 and 17 are respectively provided at their dedendums with run-off parts 71 and 73 corresponding to the addendums of tooth traces 47 and 45 of the mating rotors, to thereby define space between the run-off parts 71 and 73 and the mating tooth traces 47 and 45 respectively. The provision of the run-off parts 71 and 73 prevents the undercut of the dedendums of the rotors 15 and 17 affected by the tip edges of the tooth traces 47 and 45 respectively. The addendums of the tooth traces 45 and 47 are formed on their both sides in the rotational direction with notched parts 79 and 81 which make the space open to a rotor casing 29.