A charging device according to an embodiment includes a surface potential sensor configured to output a result of detecting a surface potential of a photoconductor, on which charge is deposited by the charging device, of an image forming apparatus and a main control unit. The main control unit is configured to perform a charging-current determining process which determines a charging-current setpoint value, which is an output setpoint value for constant current control of a charging bias to be fed from a corona power supply, based on a difference between the result of detection output from the surface potential sensor and a grid setpoint value, which is an output setpoint value for constant voltage control of a grid bias to be fed from a grid power supply, at predetermined timing.