One of the amount of induction heating by a coil and that of induction heating by coils is increased/decreased relative to the other, and the ratio of the amount of the induction heating by the coil to that of the induction heating by the coils, at which the amount of the induction heating by the coil increases and that of the induction heating by coils decreases, is set at, e.g., "9:1", and the ratio of the amount of the induction heating by the coils to that of the induction heating by the coil, at which the amount of the induction heating by the coils increases and that of the induction heating by coil decreases, is set at, e.g., "7:3".