An image forming apparatus that forms an image having proper density regardless of changes in a charged amount of developer per unit mass is provided. When image data is received in a laser printer, a main motor is driven to rotate a photosensitive drum, etc., and charging bias is applied to a charger to charge the photosensitive drum. Then, paper is fed. Predetermined current detection developing bias is applied to a developing roller for a predetermined period of time. A current value of a developing current during the application is detected. Developing bias applied to the developing roller is calculated so as to keep constant density in the printed image. Particularly, a charged amount of toner per unit mass (Q/M) is calculated from the detected current value. Based on the Q/M, the developing bias is calculated. A print process is performed by applying the developing bias to the developing roller.