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