Patent ID: 8837959

Claim:
An image forming device to print an image by receiving a developer including toner and a magnetic carrier from a storage unit, to form a developer layer on a surface of a developing roller by using the developer, and to move and adhere the toner from the developer layer to a photosensitive body by applying developing bias to the developing roller, the image forming device comprising: a toner supplying unit to supply the toner to the storage unit; a mixer to supply the developer in the storage unit to the developing roller; a developer sensor to detect the amount of developer on the surface of the developing roller; a toner concentration sensor to detect toner concentration in the developer stored in the storage unit; and a control unit to adjust at least one of a developing bias and a rotation speed of the mixer based on a value detected by the developer sensor and a value detected by the toner concentration sensor such that a printed image has uniform density, wherein the developer sensor is a capacitive sensor to detect a thickness of the developer layer, the toner concentration sensor is a magnetic sensor to detect concentration of the toner indirectly by detecting the amount of the magnetic carrier, and wherein the developer sensor includes a plurality of developer sensors arranged in a lengthwise direction of the developing roller.