Provided are a measurement device, a sensor unit, and a sensor placement method using those, which enable the suppression of the occurrence of a situation in which when a measurement is carried out with a sensor implanted under the skin, the implanted sensor becomes faulty. A sensor unit (2) is used for measuring numerical information relating to a substance contained in a body fluid in a body The sensor unit (2) is provided with a sensor (15) a part of which is placed under the skin and generates a signal according to the state of the substance, a base (10) which is disposed on the skin and holds the sensor (15), a variable mechanism (11) which is attached to the base (10) and enables the position and/or the direction of the sensor (15) with reference to the base (10) to be varied, and an external terminal (12) which is provided in the base (10) and leads the signal generated by the sensor (15) to the outside.
