PURPOSE: To perform good fixing with an optimum energy by detecting the magnetism of a toner image transferred onto a recording paper and controlling the flash energy of a flash lamp in accordance with this detection output.

CONSTITUTION: The magnetism of a toner image 2 formed on a recording paper 3 is detected by a magnetic detecting means 1. The magnetic detecting means 1 consists of a coil 6, a capacitor 8, and an oscillator 7, and the voltage between both ends of the coil 6 is rectified by a rectifying circuit 9, and a voltage V0 is inputted to a charging voltage control circuit 4 to charge a discharging capacitor 11 through a charging power supply part 10. When the charging quantity of the discharging capacitor 11 reaches a value adapted to the magnetism of the toner image 2, a flash lamp 5 is caused to emit light. Thu, good fixing is performed with an optimum energy by a simple device.