PURPOSE: To keep the heating temperature of a heated material constant by operating the speed of the heated material supplied with a feed device, according to the excess or shortage of applied voltage controlled with a power supply control device, and controlling the feed rate of the heated material on the basis of the operated value.

CONSTITUTION: The speed of a heated material 1 fed with a pinch roller 2 is operated, according to the excess or shortage of power applied from a power supply device 8 controlled with a power control device 7. An operation control device 9 controls the feed rate of the roller 2 on the basis of the operated result. Namely, the device 9 takes in currently applied power, and compares the power with the predetermined applicable power value. When there is a difference from the predetermined applicable power value, the speed of the roller 2 to match the currently applied power value is operated, and a signal showing the result of the operation is sent to a speed control device 4, thereby adjusting the speed of the roller 2 for keeping the heating temperature of the material 1 constant at all times. The material l heated up to the constant temperature is sent out from an induction heating coil 3 via the roller 2.