An image forming apparatus that can reduce the time period for rotation of an intermediate transfer unit required before toner patches are formed, and measurement of toner patch densities while continuing the rotation. A toner image formed on a photosensitive drum is transferred to a surface of the intermediate transfer unit. A toner image for detection is formed on the intermediate transfer unit, and image forming conditions are corrected based on the amount of toner obtained by reading the toner image for detection. Two density sensors read the surface of the intermediate transfer unit. Outputs from the two density sensors are stored as a first profile and a second profile, respectively. A toner density of the toner image for detection is calculated based on the first and second profiles, and image forming conditions are corrected based on the toner density.