To improve connectivity with tubes to be connected to an inlet and an outlet of a flow passage, while increasing the rapidity of an operation of changing a fluid to be passed through the flow passage and also increasing the safety. Provided is a pressure detecting device including a pressure detecting unit detecting a pressure transmitted to a diaphragm, a flow passage unit having a diaphragm for transmitting a pressure of a fluid flowing through the flow passage to the diaphragm, and a nut for removably mounting the flow passage unit onto the pressure detecting unit. The nut mounts the flow passage unit onto the pressure detecting unit when a position of a positioning groove of the pressure detecting unit about an axis coincides with a position of a positioning projection of the flow passage unit about an axis.