A pressure detector for a blood purification apparatus, comprising: a case obtained by mating a first case portion (Ca) and a second case portion (Cb) to each other, the case having a housing space inside and being connectable to a flow route for liquid; an elastic membrane as an elastic member attached to the case and with which a first housing space covered by the first case portion (Ca) and a second housing space covered by the second case portion (Cb) are separated from each other, wherein the first housing space serves as a liquid-phase portion to be supplied with the liquid in the flow route and the second housing space serves as a gas-phase portion to be supplied with gas, and wherein the elastic membrane is a membrane member with which the liquid-phase portion and the gas-phase portion are separated from each other and that is displaceable in accordance with a pressure of the liquid supplied into the liquid-phase portion; fixing parts (Q2, Q3) provided at respective peripheries of the first case portion (Ca) and the second case portion (Cb) and at which the first case portion (Ca) and the second case portion (Cb) that are mated to each other are fixed to each other; holding surfaces (m1, m2) provided at the respective peripheries of the first case portion (Ca) and the second case portion (Cb) and between which a peripheral edge of the elastic membrane is held; and a sealing part provided at the periphery of the first case portion (Ca) or the second case portion (Cb) on an inner side with respect to the fixing parts (Q2, Q3) and that seals an entirety of the peripheral edge of the elastic membrane held between the holding surfaces (m1, m2), wherein the pressure detector is configured to detect the pressure of the liquid in the flow route by detecting a pressure in the gas-phase portion, characterized in that the pressure detector has a releasing part (11, 12) that is configured to release a pressure in an air gap (α) produced between the sealing part and the fixing parts (Q2, Q3). a case obtained by mating a first case portion (Ca) and a second case portion (Cb) to each other, the case having a housing space inside and being connectable to a flow route for liquid; an elastic membrane as an elastic member attached to the case and with which a first housing space covered by the first case portion (Ca) and a second housing space covered by the second case portion (Cb) are separated from each other, wherein the first housing space serves as a liquid-phase portion to be supplied with the liquid in the flow route and the second housing space serves as a gas-phase portion to be supplied with gas, and wherein the elastic membrane is a membrane member with which the liquid-phase portion and the gas-phase portion are separated from each other and that is displaceable in accordance with a pressure of the liquid supplied into the liquid-phase portion; fixing parts (Q2, Q3) provided at respective peripheries of the first case portion (Ca) and the second case portion (Cb) and at which the first case portion (Ca) and the second case portion (Cb) that are mated to each other are fixed to each other; holding surfaces (m1, m2) provided at the respective peripheries of the first case portion (Ca) and the second case portion (Cb) and between which a peripheral edge of the elastic membrane is held; and a sealing part provided at the periphery of the first case portion (Ca) or the second case portion (Cb) on an inner side with respect to the fixing parts (Q2, Q3) and that seals an entirety of the peripheral edge of the elastic membrane held between the holding surfaces (m1, m2), wherein the pressure detector is configured to detect the pressure of the liquid in the flow route by detecting a pressure in the gas-phase portion, characterized in that the pressure detector has a releasing part (11, 12) that is configured to release a pressure in an air gap (α) produced between the sealing part and the fixing parts (Q2, Q3).