An image forming apparatus is disclosed which reads a test pattern, including a reading unit, a relative movement unit, a second detected data obtaining unit which obtains one or more second detected data sets of a reflected light which is received from a scanning position of a light by a light receiving unit while the reading unit moves relatively with respect to the recording medium before the test pattern is formed, a first detected data obtaining unit which obtains one or more first detected data sets of the reflected light which is received by the light receiving unit when the light moves over the test pattern at generally the same scanning position as the scanning position while the reading unit moves relatively with respect to the recording medium after the test pattern is formed, and a signal correction unit.