A blood purification apparatus having a predetermined attaching surface and comprising a pressure detector that includes a case connectable to a flow route for liquid and attachable to the predetermined attaching surface; and a membrane member attached inside 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 being displaceable 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, the pressure detector comprising: an inlet port including a connecting portion (C1b) connectable to the flow route for the liquid, and a flow-route portion (C1a) through which the liquid flows into an inlet opening of the liquid-phase portion; and an outlet port including a connecting portion (C2b) connectable to the flow route for the liquid, and a flow-route portion (C2a) through which the liquid having flowed into the liquid-phase portion is discharged from an outlet opening, the case includes a liquid-phase-portion case (Ca) defining the liquid-phase portion, and a gas-phase-portion case (Cb) defining the gas-phase portion; the membrane member is held between the liquid-phase-portion case (Ca) and the gas-phase-portion case (Cb) that are mated to each other, characterized in that the outlet opening is positioned in a part of the liquid-phase portion that includes a highest point in a vertical direction in a state where the case is attached to the predetermined attaching surface, and the outlet port extends obliquely upward from the outlet opening, and that the inlet port extends substantially parallel to an attaching plane (Q, Q1, Q2, Q3) defined for the membrane member while the outlet port extends at a predetermined angle with respect to the attaching plane (Q, Q1, Q2, Q3). a case connectable to a flow route for liquid and attachable to the predetermined attaching surface; and a membrane member attached inside 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 being displaceable 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, the pressure detector comprising: an inlet port including a connecting portion (C1b) connectable to the flow route for the liquid, and a flow-route portion (C1a) through which the liquid flows into an inlet opening of the liquid-phase portion; and an outlet port including a connecting portion (C2b) connectable to the flow route for the liquid, and a flow-route portion (C2a) through which the liquid having flowed into the liquid-phase portion is discharged from an outlet opening, the case includes a liquid-phase-portion case (Ca) defining the liquid-phase portion, and a gas-phase-portion case (Cb) defining the gas-phase portion; the membrane member is held between the liquid-phase-portion case (Ca) and the gas-phase-portion case (Cb) that are mated to each other, characterized in that the outlet opening is positioned in a part of the liquid-phase portion that includes a highest point in a vertical direction in a state where the case is attached to the predetermined attaching surface, and the outlet port extends obliquely upward from the outlet opening, and that the inlet port extends substantially parallel to an attaching plane (Q, Q1, Q2, Q3) defined for the membrane member while the outlet port extends at a predetermined angle with respect to the attaching plane (Q, Q1, Q2, Q3). an inlet port including a connecting portion (C1b) connectable to the flow route for the liquid, and a flow-route portion (C1a) through which the liquid flows into an inlet opening of the liquid-phase portion; and an outlet port including a connecting portion (C2b) connectable to the flow route for the liquid, and a flow-route portion (C2a) through which the liquid having flowed into the liquid-phase portion is discharged from an outlet opening, the case includes a liquid-phase-portion case (Ca) defining the liquid-phase portion, and a gas-phase-portion case (Cb) defining the gas-phase portion; the membrane member is held between the liquid-phase-portion case (Ca) and the gas-phase-portion case (Cb) that are mated to each other, characterized in that the outlet opening is positioned in a part of the liquid-phase portion that includes a highest point in a vertical direction in a state where the case is attached to the predetermined attaching surface, and the outlet port extends obliquely upward from the outlet opening, and that the inlet port extends substantially parallel to an attaching plane (Q, Q1, Q2, Q3) defined for the membrane member while the outlet port extends at a predetermined angle with respect to the attaching plane (Q, Q1, Q2, Q3).