PURPOSE: To enable reflection on processing conditions of a magnetic material with a quick determination of a correct iron loss value by reducing a measuring coil through which a material to be measured running in a similar manner to prepare a mini-coil for correction made smaller in side.

CONSTITUTION: In a measuring circuit, mini-coils 102, 103 and 104 for correction made by being reduced to a size of about 1/10 are arrange to connect freely. Current of an exciting coil made piercing an electromagnetic steel plate is inputted into a watt meter of a measuring circuit together with an induced voltage of a detection coil and a reading of the watt meter under a constant magnetization is divided by the weight of the electromagnetic steel plate to measure an iron loss value. Here, the measuring circuit is connected to the mini coils for calibration into which a sample material is inserted with an Epstein value known and a measuring accuracy of an iron loss continuously measuring device is checked comparing the current output with a measured value of the iron loss continuously measuring device. Thus, an iron loss measurement is accomplished accurately and quickly.