There is provided an image forming apparatus which is capable of securing a time period for measuring the base reflected light quantity required for the base correction, and at the same time, reducing a time period required for the entire image density control. An image forming unit includes an image carrier disposed to be exposed to light to have a latent image formed thereon, an electrostatic charger that charges the image carrier to a predetermined polarity, a developing device that visualizes the latent image formed on the image carrier to form a visible image, and an endless belt onto which the visible image is transferred. A CPU controls the image forming unit to form predetermined detection patterns on the endless belt. The detection patterns and the quantity of reflection light from the endless belt are detected. The CPU corrects the detected detection patterns based on the detected quantity of reflection light. One of the image forming conditions is adjusted by the CPU, based on the corrected detection result of the detection patterns. Another one of the image forming conditions is adjusted by the CPU. The detection of the quantity of reflection light from the endless belt is carried out in timing synchronous with the adjustment of the other image forming condition.