A roots pump including a pair of multi-lobe rotors is disclosed. The pair of rotors each include a first straight portion and a second straight portion. The first straight portion is provided on one of end portions of the rotary shaft of the rotor in an axial direction to extend straight along the axial direction. The straight portion is provided on the other one of the end portions to extend straight along the axial direction. The phases of the first straight portion and the second straight portion are displaced along the circumferential direction of the associated rotary shaft. Each of the pair of rotors further includes a coupling portion, which couples the first straight portion and the second straight portion. The coupling portions of the pair of rotors are engaged with each other to suppress fluid leakage between the pair of rotors.