To provide a head opening sealing gasket which can be safely slid to a proper position within the barrel casing of a barrel compressor.

A head opening sealing gasket used for a barrel compressor having a head fitted into a barrel casing has a curved outer surface 60 according to the curvature of the bore of the barrel casing. An O-ring is fitted into an O-ring groove 62 formed on this outer surface, and engaged with the inner surface of the barrel casing in the sealed state. The head opening sealing gasket is received within an opening formed on the outside cylindrical surface of the head. The head and the head opening sealing gasket mounted thereon can be slid to a proper position within the barrel casing by the chamfered part of the barrel casing without being accompanied with the damage of the O-ring.