An image forming apparatus is able to form steady test-pattern toner images for calibration of reflection density detection characteristics with ample toner. The operation consists of forming a test-pattern toner image for calibration of the reflection density detection characteristics, which is formed of a plurality of toner images of different colors aside from a reference test-pattern toner image for controlling image forming conditions, detecting the detection characteristics of a reflection density sensor from the result of detection of the quantity of toner attached to the test-pattern toner image for calibration of the reflection density detection characteristics, and calibrating the output characteristics of the reflection density sensor.