An image forming apparatus is disclosed which reads a test pattern formed by ejecting liquid droplets onto a recording medium to adjust an ejection timing of the liquid droplets. The image forming apparatus includes a reading unit; a relative movement unit; a second detected data obtaining unit; a first detected data obtaining unit; a subtraction processing unit which subtracts a value comparable to a local minimum value of first detected data sets from each of the first detected data sets and second detected data sets; and a signal correction unit which calculates a proportion of the subtracted first detected data sets relative to the subtracted second detected data sets to align a local maximum value of the first detected data sets such that it is generally constant.