An image forming apparatus that is capable of preventing an abnormal operation due to a voltage drop of a commercial power source and continuing an image forming operation efficiently even if the voltage drop occurs. A fixing unit fixes a toner image transferred onto a sheet. A voltage detection unit detects an input voltage of the commercial power source. A setting unit sets a fixing electric power supplied to the fixing unit from the commercial power source. A control unit suspends an operation when the set fixing electric power is less than a predetermined electric power and when the input voltage is less than a first voltage, and to continue the operation while reducing the set fixing electric power when the set fixing electric power is not less than the predetermined electric power and when the input voltage is less than a second voltage.