A developing device includes: a developing roller that carries a developer; a developing tank that supplies the developing roller with the developer replenished from a replenishing port; a discharger that has a discharge port connecting to the developing tank and discharges the developer from the developing tank; a supply amount acquirer that acquires a supply amount of the developer to be supplied to the developing tank; a discharge amount acquirer that acquires a discharge amount of the developer to be discharged through the discharge port; a developer amount adjuster that adjusts an amount of the developer in the developing tank; and a hardware processor that controls the developer amount adjuster to adjust the amount of the developer in the developing tank to a value within a predetermined range, in accordance with the acquired supply amount and the acquired discharge amount of the developer.