An image forming apparatus comprising: image forming means (120-123) for forming a test chart on a sheet; conveying means for conveying the sheet; and a color sensor for measuring the test chart on the sheet conveyed by the conveying means, characterized in that : the test chart includes a first test pattern and a second testy pattern, the second test pattern being brighter than the first test pattern, and the color sensor comprises a light receiving element for receiving reflected light from the first test pattern based on a first accumulation time and receiving reflected light from the second test pattern based on a second accumulation time, wherein: the first accumulation time is a length of time for which the light receiving element accumulates the reflected light from the first test pattern, the second accumulation time is a length of time for which the light receiving element accumulates the reflected light from the second test pattern, the length of the second accumulation time is shorter than the length of the first accumulation time, and in the conveyance direction of the sheet, the length of the second test pattern is shorter than the length of the first test pattern. image forming means (120-123) for forming a test chart on a sheet; conveying means for conveying the sheet; and a color sensor for measuring the test chart on the sheet conveyed by the conveying means, characterized in that : the test chart includes a first test pattern and a second testy pattern, the second test pattern being brighter than the first test pattern, and the color sensor comprises a light receiving element for receiving reflected light from the first test pattern based on a first accumulation time and receiving reflected light from the second test pattern based on a second accumulation time, the test chart includes a first test pattern and a second testy pattern, the second test pattern being brighter than the first test pattern, and the color sensor comprises a light receiving element for receiving reflected light from the first test pattern based on a first accumulation time and receiving reflected light from the second test pattern based on a second accumulation time, wherein: the first accumulation time is a length of time for which the light receiving element accumulates the reflected light from the first test pattern, the second accumulation time is a length of time for which the light receiving element accumulates the reflected light from the second test pattern, the length of the second accumulation time is shorter than the length of the first accumulation time, and in the conveyance direction of the sheet, the length of the second test pattern is shorter than the length of the first test pattern. the first accumulation time is a length of time for which the light receiving element accumulates the reflected light from the first test pattern, the second accumulation time is a length of time for which the light receiving element accumulates the reflected light from the second test pattern, the length of the second accumulation time is shorter than the length of the first accumulation time, and in the conveyance direction of the sheet, the length of the second test pattern is shorter than the length of the first test pattern. <EOS>