A pressure detector that includes a case connectable to a flow route for liquid, and a membrane member provided in the case and with which a liquid-phase portion to be supplied with the liquid in the flow route and a gas-phase portion to be supplied with gas are separated from each other, the membrane member serves as a diaphragm provided in the case and is made of a flexible material being displaceable or deformable in accordance with a pressure of the liquid supplied to the liquid-phase portion, the pressure detector detecting the pressure of the liquid in the flow route by detecting a pressure in the gas-phase portion, wherein the gas-phase portion has an opening through which the gas is allowed to be introduced or discharged in accordance with the displacement of the membrane member, and secured portions secured for the introduction or discharge of the gas through the opening during the displacement of the membrane member toward a side of the gas-phase portion, wherein the secured portions are passages communicating with the opening and are secured for the introduction or discharge of the gas through the opening such that the gas is allowed to flow through the passages during the displacement of the membrane member toward the side of the gas-phase portion, wherein a recess is surrounding the opening and characterized in that the passages are defined by a plurality of ribs in the recess provided around the opening and projecting radially about the opening of the gas-phase portion, and wherein in a state where the membrane member displaced toward the side of the gas-phase portion is in contact with the ribs, the passages are provided as spaces provided around the opening and communicating with the opening. a case connectable to a flow route for liquid, and a membrane member provided in the case and with which a liquid-phase portion to be supplied with the liquid in the flow route and a gas-phase portion to be supplied with gas are separated from each other, the membrane member serves as a diaphragm provided in the case and is made of a flexible material being displaceable or deformable in accordance with a pressure of the liquid supplied to the liquid-phase portion, the pressure detector detecting the pressure of the liquid in the flow route by detecting a pressure in the gas-phase portion, wherein the gas-phase portion has an opening through which the gas is allowed to be introduced or discharged in accordance with the displacement of the membrane member, and secured portions secured for the introduction or discharge of the gas through the opening during the displacement of the membrane member toward a side of the gas-phase portion, wherein the secured portions are passages communicating with the opening and are secured for the introduction or discharge of the gas through the opening such that the gas is allowed to flow through the passages during the displacement of the membrane member toward the side of the gas-phase portion, wherein a recess is surrounding the opening and characterized in that the passages are defined by a plurality of ribs in the recess provided around the opening and projecting radially about the opening of the gas-phase portion, and wherein in a state where the membrane member displaced toward the side of the gas-phase portion is in contact with the ribs, the passages are provided as spaces provided around the opening and communicating with the opening.