An image forming apparatus includes: a calculating device configured to, in a testing mode to replace a discharge path between a charging wire and a grid of a charger with a testing resistor having a predetermined resistance, calculate a charge voltage using the resistance of the testing resistor and a detection value by a grid-current detecting circuit; and a controller configured to, in an image forming mode to actually form an image, detect the charge voltage from a feedback voltage using a relation between a value of the feedback voltage which is detected by a charge-voltage detecting circuit in the testing mode and the charge voltage calculated by the calculating device.