A pressure sensor is provided with a housing which has an opening portion for communicating an interior of the housing with an exterior, and a pressure detecting unit which has a detection surface for detecting a pressure applied thereto through the opening and is arranged in the housing. One of a perimeter of the detection surface and that of the opening portion has a protrusion portion which protrudes toward other of the perimeters. The other of the perimeters has a groove, into which a tip of the protrusion portion is inserted. A seal member is arranged between the groove and the protrusion portion, to seal therebetween and buffer stress caused by relative displacement between the housing and the pressure detecting unit.