A mode controller outputs a characteristic value according to a DC component of a developing current measured by an ammeter at a predetermined measurement timing. The measurement timing is defined as a timing at which a non-image forming region of a surface of a photosensitive drum is located opposite to a developing roller in the entirety of an axial direction and an electric field in a direction in which a toner moves from the photosensitive drum toward the developing roller by a potential difference between a surface potential of the photosensitive drum and the DC component of a developing bias is formed in a developing nip part. A determining section determines an execution timing for a charge amount acquisition operation according to the characteristic value output by the mode controller.