Image forming apparatus having simple configuration and capable of measuring toner current included in developing current, and accurately calculating toner charge amount based on measurement result

An image forming apparatus, comprising:

• an image forming unit that comprises:
• an image carrier having a photosensitive layer formed on the surface thereof,
• a charging device that charges the image carrier;
• an exposing device that forms an electrostatic latent image by exposing the image carrier charged by the charging device;
• and
• a developing device that has a developer carrier arranged facing the image carrier and carrying a two-component developer that includes a magnetic carrier and toner, and forms a toner image by adhering the toner to the electrostatic latent image formed on the image carrier;
• a developing voltage power supply that applies a developing voltage obtained by superimposing an AC voltage on a DC voltage on the developer carrier;
• a density detecting device that detects a density of the toner image formed by the developing device;
• a current detecting unit that detects a DC component of a developing current that flows when the developing voltage is applied to the developer carrier;
• and
• a control unit that controls the image forming unit and the developing voltage power supply;
• the control unit,
• by the developing device at a time of non-image formation, forms a reference image on the image carrier;
• by a current detecting unit, detects the developing current when the reference image is formed;
• calculates a toner current flowing due to movement of the toner by subtracting a carrier current flowing through the magnetic carrier from the detected developing current;
• calculates a toner developing amount when the reference image is formed from a density of the reference image detected by the density detecting device;
• and
• estimates the toner charge amount based on the toner current and the toner developing amount.