A method of measuring water content in a mouth comprising a means of measuring water content by bringing a sensor unit for sensing the water content of a water content measuring instrument into contact directly or via a plastic film with a part to be measured such as a tongue mucous membrane, a cheek mucous membrane and a palate. An instrument for measuring water content in a mouth comprising a sensor unit for sensing water content by being brought into contact directly or via a plastic film with a tongue mucous membrane, a cheek mucous membrane or a palate, and a measuring unit provided with this sensor unit. The sensor unit is provided with an electrostatic capacity type sensor for measuring water content by a change in permittivity. The sensor unit and the measuring unit are formed integrally, with a dogleg-curved probe provided between them. The probe is covered with a bag-like plastic-film cover starting at the sensor unit, and the bag-like cover is replaced every time a measurement is made. The bag-like cover is a replacement cover for the part to be inserted into the mouth of the in-mouth water content measuring instrument.
