PURPOSE: To heat up all iron cores to a prescribed temp. range without using a dummy material and to stably and continuously anneal the iron cores by controlling the output of an inverter for heating to the optimum condition in an annealing furnace by the input of means for detecting an annealing rate and position and calculation of time.

CONSTITUTION: The top position of the iron cores 1 is detected by an optical or electrical position detecting means 6 near an inlet of a heating coil 2 and the moving speed of the iron cores 1 is measured by a means for detecting the rotating speed of a motor for driving a conveyor 3 or other means. The position where the iron cores 1 pass in the coil 2 is calculated from the above-mentioned speed and time by a microcomputer 7. The output of the ratio determined in relation to the target output of the inverter 9 for heating at the time of the stationary operation when the inside of the furnace is full of the iron cores is gradually applied to the coil 2 when the top iron core 1 arrives at the point L1 to enter the coil 2. The stationary operation is started from the point of the time past the L2 at which the inside of the furnace is full of the iron cores. Conversely, the tail end of the iron core 1 is detected and the output is successively decreased reverse from the above when the inside of the furnace is successively emptied. The individual heating of the iron cores 1 to an adequate temp. is thereby permitted even if the inside of the furnace is not full.