A turbo-molecular pump has a housing with intake and exhaust ports. A rotary body is enclosed in the housing and has rotor blades of a series of stages formed so that the outside diameter of at least one stage on the exhaust port side is smaller than that on the intake port side. Stator blades are arranged between the rotor blades and spacer rings are arranged between the stator blades to hold the stator blades at predetermined intervals. The spacer rings are formed so that the smallest inside diameter of at least one stage of the spacer rings on the exhaust port side is smaller than the largest outside diameter of the rotor blades. The housing comprises a casing whose inside diameter is constant or whose inside diameter is smaller at the intake port side than at the exhaust port side.