A pressure detection device comprising: a flow passage unit in which a flow passage for introducing a fluid is formed; a pressure detection unit configured to detect a pressure of the fluid; and a mounting mechanism by which the flow passage unit is detachably mounted on the pressure detection unit, wherein the flow passage unit includes: a pressure receiving diaphragm configured to be displaced upon receiving the pressure of the fluid flowing through the flow passage on a first surface of the pressure receiving diaphragm; and a first connection portion attached to a second surface of the pressure receiving diaphragm, the pressure detection unit includes: a pressure detecting diaphragm having a first surface to which a pressure is to be transmitted from the first connection portion; a second connection portion attached to a center portion of a second surface of the pressure detecting diaphragm; and four strain resistance portions attached to the second surface of the pressure detecting diaphragm, the strain resistance portions being connected to each other so as to form a Wheatstone bridge circuit, one of either the first connection portion or the second connection portion is formed of a magnet, and the other of either the first connection portion or the second connection portion is formed of a magnet or a magnetic body, the first connection portion and the second connection portion are arranged such that the first connection portion and the second connection portion are attracted by a magnetic force in a state where the flow passage unit is mounted on the pressure detection unit by the mounting mechanism, and the four strain resistance portions are attached to a region of the second surface of the pressure detecting diaphragm other than the center portion. a flow passage unit in which a flow passage for introducing a fluid is formed; a pressure detection unit configured to detect a pressure of the fluid; and a mounting mechanism by which the flow passage unit is detachably mounted on the pressure detection unit, wherein the flow passage unit includes: a pressure receiving diaphragm configured to be displaced upon receiving the pressure of the fluid flowing through the flow passage on a first surface of the pressure receiving diaphragm; and a first connection portion attached to a second surface of the pressure receiving diaphragm, a pressure receiving diaphragm configured to be displaced upon receiving the pressure of the fluid flowing through the flow passage on a first surface of the pressure receiving diaphragm; and a first connection portion attached to a second surface of the pressure receiving diaphragm, the pressure detection unit includes: a pressure detecting diaphragm having a first surface to which a pressure is to be transmitted from the first connection portion; a second connection portion attached to a center portion of a second surface of the pressure detecting diaphragm; and four strain resistance portions attached to the second surface of the pressure detecting diaphragm, the strain resistance portions being connected to each other so as to form a Wheatstone bridge circuit, a pressure detecting diaphragm having a first surface to which a pressure is to be transmitted from the first connection portion; a second connection portion attached to a center portion of a second surface of the pressure detecting diaphragm; and four strain resistance portions attached to the second surface of the pressure detecting diaphragm, the strain resistance portions being connected to each other so as to form a Wheatstone bridge circuit, one of either the first connection portion or the second connection portion is formed of a magnet, and the other of either the first connection portion or the second connection portion is formed of a magnet or a magnetic body, the first connection portion and the second connection portion are arranged such that the first connection portion and the second connection portion are attracted by a magnetic force in a state where the flow passage unit is mounted on the pressure detection unit by the mounting mechanism, and the four strain resistance portions are attached to a region of the second surface of the pressure detecting diaphragm other than the center portion.