In an image forming apparatus, the number of components and a size of the apparatus is reduced by realizing both functions for detecting a toner patch image to perform image quality adjustment and for performing precise color adjustment depending on a sheet color with one color sensor. The color sensor is disposed in a vicinity of a transport belt, a reference toner image is formed on the transport belt at a predetermined timing. A color of the reference toner image formed on the belt is detected by the color sensor to adjust development conditions. Further, the sheet is transported in a state where the transport belt is separated from a photoreceptor drum, and the sheet color is detected by the color sensor, and thereafter the transport belt touches the photoreceptor drum to transport the sheet to a transfer portion.