In an embodiment, a contact pivoting plate, which comes into contact with a developer swelled by a discharge mixer, is provided in a case. The contact pivoting plate is pivoted according to a detection result of a temperature and humidity sensor to change the magnitude of resistance by the contact pivoting plate with respect to a flow of the developer. The height of the swell of the developer is suppressed from fluctuating according to an environmental change. The fluctuation in an amount of an excess developer discharged from a discharge port is suppressed to replace a deteriorated carrier in a development container with a new carrier little by little.