An image forming apparatus comprising: one or a plurality of photosensitive drums (44K to 44C); a plurality of chargers (41K to 41C) each having a grid (43K to 43C), which are provided for the one photosensitive drum or are respectively provided for the plurality of photosensitive drums and charge the one or plurality of photosensitive drums; a voltage applying unit configured to generate a charging voltage and to apply the generated charging voltage to the plurality of chargers in common; a plurality of grid constant voltage circuits (71K to 71C) which are respectively provided for the plurality of chargers, each of the plurality of grid constant voltage circuits including: a voltage detecting unit (73K to 73C) configured to detect a voltage based on a grid voltage (GRID1) which is generated in the grid (43K to 43C) in accordance with the applied charging voltage; a first current detecting unit (74K to 74C) configured to detect a first current flowing into the voltage detecting unit; a voltage control line (Ln) configured to make the grid voltage constant; and an operation control device configured to perform a feedback control through the voltage control line (Ln) such that the detected voltage detected by the voltage detecting unit has a predetermined voltage value (Vth); at least one of a second current detecting units (72K to 72C) which are respectively provided for at least one of the grid constant voltage circuits, and are configured to detect a second current flowing into the voltage control line; and a controller (ASIC 51) configured to control the voltage applying unit such that a sum of the first current and the second current corresponding to one of the plurality of chargers becomes a predetermined current value. one or a plurality of photosensitive drums (44K to 44C); a plurality of chargers (41K to 41C) each having a grid (43K to 43C), which are provided for the one photosensitive drum or are respectively provided for the plurality of photosensitive drums and charge the one or plurality of photosensitive drums; a voltage applying unit configured to generate a charging voltage and to apply the generated charging voltage to the plurality of chargers in common; a plurality of grid constant voltage circuits (71K to 71C) which are respectively provided for the plurality of chargers, each of the plurality of grid constant voltage circuits including: a voltage detecting unit (73K to 73C) configured to detect a voltage based on a grid voltage (GRID1) which is generated in the grid (43K to 43C) in accordance with the applied charging voltage; a first current detecting unit (74K to 74C) configured to detect a first current flowing into the voltage detecting unit; a voltage control line (Ln) configured to make the grid voltage constant; and an operation control device configured to perform a feedback control through the voltage control line (Ln) such that the detected voltage detected by the voltage detecting unit has a predetermined voltage value (Vth); a voltage detecting unit (73K to 73C) configured to detect a voltage based on a grid voltage (GRID1) which is generated in the grid (43K to 43C) in accordance with the applied charging voltage; a first current detecting unit (74K to 74C) configured to detect a first current flowing into the voltage detecting unit; a voltage control line (Ln) configured to make the grid voltage constant; and an operation control device configured to perform a feedback control through the voltage control line (Ln) such that the detected voltage detected by the voltage detecting unit has a predetermined voltage value (Vth); at least one of a second current detecting units (72K to 72C) which are respectively provided for at least one of the grid constant voltage circuits, and are configured to detect a second current flowing into the voltage control line; and a controller (ASIC 51) configured to control the voltage applying unit such that a sum of the first current and the second current corresponding to one of the plurality of chargers becomes a predetermined current value.