An image forming apparatus comprising: a plurality of photosensitive drums (41B, 41Y, 41M, 41C); a plurality of scorotron chargers (50B, 50Y, 50M, 50C) which are provided for the plurality of photosensitive drums, respectively, the plurality of scorotron chargers for charging the photosensitive drums, respectively; a voltage application circuit including an output circuit; a plurality of wires which are provided for the plurality of scorotron chargers, respectively; a plurality of grid electrodes which are provided for the plurality of scorotron chargers, respectively; at least one of current detecting units (260B, 260Y, 260M, 260C) which are provided for at least one of the plurality of grid electrodes, respectively, and at least one of the current detecting units for detecting grid current (Ig) which flows into at least one of the grid electrodes, respectively; and a control device; characterized in that: the voltage application circuit is commonly connected to the plurality of scorotron chargers, and configured to apply a high voltage to the plurality of scorotron chargers, the wires of the individual scorotron chargers (50B, 50Y, 50M) being commonly connected to an output line (Lo) of the output circuit whereby an output voltage (Vo) of the output circuit is applied to the wires of the individual chargers (50B, 50Y, 50M, 50C); and the control device is configured to control the voltage application circuit such that at least one of the grid currents (Ig) detected by at least one of the current detecting units become equal to or higher than a reference value; wherein a plurality of the current detecting units (260B, 260Y, 260M, 260C) are provided for the plurality of grid electrodes, respectively, the plurality of the current detecting units for performing detections of the grid currents, respectively, and the control device is configured to discriminate a minimum current value from the detected grid currents, and to control the voltage application circuit such that a grid current of the minimum current value becomes a constant current which is equal to or higher than the reference value. a plurality of photosensitive drums (41B, 41Y, 41M, 41C); a plurality of scorotron chargers (50B, 50Y, 50M, 50C) which are provided for the plurality of photosensitive drums, respectively, the plurality of scorotron chargers for charging the photosensitive drums, respectively; a voltage application circuit including an output circuit; a plurality of wires which are provided for the plurality of scorotron chargers, respectively; a plurality of grid electrodes which are provided for the plurality of scorotron chargers, respectively; at least one of current detecting units (260B, 260Y, 260M, 260C) which are provided for at least one of the plurality of grid electrodes, respectively, and at least one of the current detecting units for detecting grid current (Ig) which flows into at least one of the grid electrodes, respectively; and a control device; characterized in that: the voltage application circuit is commonly connected to the plurality of scorotron chargers, and configured to apply a high voltage to the plurality of scorotron chargers, the wires of the individual scorotron chargers (50B, 50Y, 50M) being commonly connected to an output line (Lo) of the output circuit whereby an output voltage (Vo) of the output circuit is applied to the wires of the individual chargers (50B, 50Y, 50M, 50C); and the control device is configured to control the voltage application circuit such that at least one of the grid currents (Ig) detected by at least one of the current detecting units become equal to or higher than a reference value; wherein the voltage application circuit is commonly connected to the plurality of scorotron chargers, and configured to apply a high voltage to the plurality of scorotron chargers, the wires of the individual scorotron chargers (50B, 50Y, 50M) being commonly connected to an output line (Lo) of the output circuit whereby an output voltage (Vo) of the output circuit is applied to the wires of the individual chargers (50B, 50Y, 50M, 50C); and the control device is configured to control the voltage application circuit such that at least one of the grid currents (Ig) detected by at least one of the current detecting units become equal to or higher than a reference value; wherein a plurality of the current detecting units (260B, 260Y, 260M, 260C) are provided for the plurality of grid electrodes, respectively, the plurality of the current detecting units for performing detections of the grid currents, respectively, and the control device is configured to discriminate a minimum current value from the detected grid currents, and to control the voltage application circuit such that a grid current of the minimum current value becomes a constant current which is equal to or higher than the reference value.