a pump includes a pump body defining first and second cavities . first and second rotors are disposed in the first and second cavities between an inlet passage and an outlet passage and are in sealing relationship with each other during rotation of the rotors . a drive transmission mechanism is coupled to the first and second rotors for rotating the rotors in opposite directions . during a revolution of the first rotor , a vane projecting from the first rotor emerges from a recess of the second rotor , passes the inlet passage and the outlet passage , and enters a recess of the second rotor , while the inlet passage remains sealed from the outlet passage .