In a vacuum pump, an outer diameter of a spiral plate disposed on a downstream side is set smaller than an outer diameter of a spiral plate disposed on an upstream side. Specifically, a stepped portion is provided by setting a blade length of the spiral plate disposed on the downstream side shorter than a blade length of the spiral plate disposed on the upstream side. In addition, in a spacer provided in the stepped portion, a relief formation portion is provided to allow a contact surface in contact with an upstream spacer (i.e., spacer opposed to the spiral plate having the unreduced outer diameter) and a contact surface in contact with a downstream spacer (i.e., spacer opposed to the spiral plate having the reduced outer diameter) in the stepped portion to have an equal inner diameter.