PURPOSE: To easily measure the iron loss of outer core including all effects by measuring rotatory force induced in the outer core.

CONSTITUTION: If there is iron loss in an outer core 4 used in a rotating magnetic field, rotatory force is induced in the outer core 4. By letting the iron loss W (W), the induced rotatory force T (Nm), rotation number of the rotating magnetic field n (rpm) and the ratio of the circumference of a circle to its diameter π, W is expressed as W=2πnT/60, and the iron loss of a magnetic material in a rotating magnetic field is measured only from the induced rotational force. The rotational force is not related with the outer core conditions, it can be evaluated in actually used state including all effects of shape, distortion, magnetic flux distribution, centrifugal force, plate number in the case of plate, layering method, fixing method, etc. For constitution, the two outer cores 4 as the sample are layered and fixed with a sample holder 8, which is rotated with an angle velocity of n0 in the rotating magnetic field of rotation number n+n0 given by three-phase power 1, magnetic field generation coil 2 and iron core 3, and induced rotatory force T and the rotation number n are detected 5.