PURPOSE:To always form an exact image by detecting a state of a high-voltage generating means for electrostatically charging a photosensitive drum and detecting its abnormality in accordance with the detected output. CONSTITUTION:Voltage of a primary high voltage generator 21 is applied to the primary charger 2 of an image forming device to charge a photosensitive drum. Voltage generated with the secondary coil 32 of the transformer 30 of the generator 21 is rectified and smoothed and applied to the charger 2. The base emitter of a transistor (TR)Q2 is connected through a resistor R1 to the secondary coil 33, and the collector is connected through a signal conductor 21A to a microcomputer 15. TRQ2 is turned on and off in accordance with frequency, synchronism, or pulse width of output voltage induced on the coil 33. As a result, the computer 15 detects abnormality of the generator 21 and displays it, and it can change control of the generator 21 to always execute exact image formation.