A pressure sensor comprising: a pressure sensor element displaceable by a pressure of a fluid to be measured introduced through a connected member; a metal adapter integrally attached to the pressure sensor element, the adapter defining therein a hole through which the pressure of the fluid to be measured is introduced to the pressure sensor element; a metal fitting member provided with a housing recess for receiving the adapter, the fitting member being connectable to the connected member; and an operation member for pressing a valve provided to the connected member, the operation member defining a communicating path through which a flow path of the connected member, in which the pressure of the fluid to be measured is introduced, is in communication with the hole of the adapter, characterized in that the operation member being a synthetic resin member comprising: a contact portion brought into contact with the adapter; and a pressing portion provided on a side opposite to the contact portion to press the valve, wherein at least one of the adapter and the fitting member is adapted to bite into the other one of the adapter and the fitting member to be airtightly attached with the other one of the adapter and the fitting member due to a plastic deformation of a surface of the one of the adapter and the fitting member facing the other one of the adapter and the fitting member in a direction orthogonal to an axial direction. a pressure sensor element displaceable by a pressure of a fluid to be measured introduced through a connected member; a metal adapter integrally attached to the pressure sensor element, the adapter defining therein a hole through which the pressure of the fluid to be measured is introduced to the pressure sensor element; a metal fitting member provided with a housing recess for receiving the adapter, the fitting member being connectable to the connected member; and an operation member for pressing a valve provided to the connected member, the operation member defining a communicating path through which a flow path of the connected member, in which the pressure of the fluid to be measured is introduced, is in communication with the hole of the adapter, characterized in that the operation member being a synthetic resin member comprising: a contact portion brought into contact with the adapter; and a pressing portion provided on a side opposite to the contact portion to press the valve, wherein at least one of the adapter and the fitting member is adapted to bite into the other one of the adapter and the fitting member to be airtightly attached with the other one of the adapter and the fitting member due to a plastic deformation of a surface of the one of the adapter and the fitting member facing the other one of the adapter and the fitting member in a direction orthogonal to an axial direction. a contact portion brought into contact with the adapter; and a pressing portion provided on a side opposite to the contact portion to press the valve, wherein at least one of the adapter and the fitting member is adapted to bite into the other one of the adapter and the fitting member to be airtightly attached with the other one of the adapter and the fitting member due to a plastic deformation of a surface of the one of the adapter and the fitting member facing the other one of the adapter and the fitting member in a direction orthogonal to an axial direction.