PURPOSE: To perform a wide range of measurement with a simple constitution by adjusting and heating a sensor element composed of a shape memory material to an operating temperature which falls within the structural phase transformation region of the sensor material.

CONSTITUTION: A sensor element 1 is formed of a shape memory material and, when its operating temperature falls within a temperature range in which the shape and length of the element 1 change from the shape memory characteristic of the material, fulfills a desired function. A monitoring element 19 transmits an electric signal which varies depending upon the length, temperature, or tensile force of the element 1 to a comparison amplifier 25. A desired target value is also supplied to the amplifier 25 from a target value generator 26. The length, temperature, and tensile force of the element 1 are maintained constantly regardless of the ambient temperature by controlling an adjustable resistor 23 by using these signals. Therefore, an electric current which can be detected by means of a current measuring instrument and flows through a heating current circuit 21 is obtained as a scale of external influence.