An image forming apparatus capable of performing color misregistration correction at image forming speeds while reducing downtime. Image forming units form images of different colors. The formed images are transferred to an intermediate transfer belt. A pattern sensor detects color misregistration amounts of color patterns formed on the intermediate transfer belt. A CPU causes the image forming units to form first color patterns at a first speed and the pattern sensor to detect first color misregistration amounts, and causes the former to form second color patterns at a second speed different from the first speed and the latter to detect second color misregistration amounts. The CPU determines third color misregistration amounts at a third speed based on the first and second color misregistration amounts. The third speed is different from the first and second speeds.