A Roots pump has a driving shaft press fitted into a driving rotor, and a driven shaft press fitted into a driven shaft. A driving timing gear is located between an electric motor and the driving rotor. The driving rotor has a driving assist shaft that projects in a direction opposite to the driving timing gear. The specific gravity of the material of the driving assist shaft is less than the specific gravity of the material of the driving rotary shaft. A driven rotor has a driven assist shaft that projects in a direction opposite to a driven timing gear. The specific gravity of the material of the driven assist shaft is less than the specific gravity of the material of the driven rotary shaft.