Abstract: To suck and discharge the air into and from a pressure reducing chamber of a vacuum pump, a check valve comprises a valve body made of plastic material in a mushroom configuration, and a plate member which includes a flat valve seat surface to be engaged with the annular rim of the cap portion of the valve body, an assembling hole to be coupled with the neck portion of the valve body, and a plurality of openings positioned symmetrically on a circle concentric with the assembling hole to normally be closed by the cap portion of the valve body. The neck portion of the valve body is assembled with the assembling hole under axially and radially compressed condition to air-tightly engage the annular rim of the cap portion of the valve body on the valve seat surface.