A pressure sensor includes: a pressure sensor element; an adapter integrally attached to the pressure sensor element and defining therein a hole where a pressure of a fluid to be measured is introduced to the pressure sensor element; a fitting member having a housing recess housing the adapter and connectable to a connected member; and a pressing member pressing a valve of the connected member and defining a communicating path through which a flow path where the pressure of the fluid is introduced is in communication with the hole of the adapter. The fitting member is made of a synthetic resin. An O-ring is interposed between a circumferential surface of the adapter and the fitting member. The adapter is a synthetic resin member, and includes a contact portion brought into contact with the adapter and a pressing portion pressing the valve. The pressing member is welded to the fitting member.