An warming-up operation is performed by driving the first coil and the second coil in accordance in an operating mode already stored, until the temperatures T1 and T2 detected by the first and second temperature sensors, respectively, reach preset value Ts. A temperature rise ΔT1 per unit time of the temperature T1 detected by the first temperature sensor during the warming-up operation is determined. Also, temperature rise ΔT2 per unit time of the temperature T2 detected by the second temperature sensor during the warming-up operation is determined. The operating mode is updated to such an operating mode that the temperature rises ΔT1 and ΔT2 determined by the second control section become equal to each other.