A flow passage unit is detachably mounted on a pressure detection unit and detects a pressure to be transmitted to a pressure detection part. The flow passage unit includes a flexible flow passage part including a flexible flow passage which introduces liquid, flowing in the flexible flow passage part from an inflow port, to an outflow port, and which is made of a flexible material. The flow passage unit also includes a body portion configured to form a pressure transmitting space which surrounds an outer peripheral surface of the flexible flow passage part. The flow passage unit also includes a pressure transmitting part mounted on the body portion and having a thin-film shape. One surface of the pressure transmitting part faces the pressure transmitting space, and the other surface of the pressure transmitting part is capable of coming into contact with the pressure detection part.