PURPOSE: To enable measurement of the absolute value of tensile strain and compression strain by pressing the magnetic poles of a U-shaped iron core with input and output coils wound round it upon a surface whereon the measurement is performed in such a manner that the poles are positioned at a right angle to the loading direction of a metal material to be measured.

CONSTITUTION: Magnetic poles 4 and 5 are formed by winding an input coil 1 and an output coil 2 round the U-shaped iron core 3. The coil 1 is connected with an oscillator which gives the alternate voltage of proper frequency thereto, while the coil 2 is connected with a detector for detecting the output. The magnetic poles 4 and 5 are pressed on a surface 7 whereon the measurement is performed in such a manner that the U-shaped plane of the iron core 3 has the direction B-B perpendicular to the loading direction A-A. By this constitution, the magnetic permeability of a metal material 6 changes rectilineally in proportion to the magnitude of the stress generated through the change in the tensile load as shown in the figure. The compression load also brings about similar change, although the direction thereof is reverse. Therefore, the absolute values of tensile strain and compression strain themselves can be measured.