Image forming device, developer stirring method and non-transitory recording medium

An image forming device to and from which a developing unit in which a developer containing toner is filled is detachable and removable, comprises a hardware processor that: enables the developing unit to perform a stir operation of the developer filled in the developing unit as an initial operation of the developing unit; obtains a reference value of a toner density set in advance in time of a factory shipment of the developing unit at time the stir operation is performed; and compares a toner density received from the developing unit during the stir operation with the reference value to determine a stir condition of the developer.