A repetitive object detecting device includes data-retention and difference-calculation units, an adding unit, a horizontal direction accumulating unit and a small and large comparing unit. Each data-retention and difference-calculation unit carries out, with respect to a plurality of lines, a process for setting as reference pixel data pixel data located at an end of a plurality of pieces of pixel data, and calculating a difference between the reference pixel data and pixel data separated from the reference pixel data by k pixels (2≦k≦maximum number) to obtain difference data by each separated pixel number k. The adding unit adds the difference data in the plurality of lines by each separated pixel number k. The horizontal direction accumulating unit accumulates the added data by each separated pixel number k in a horizontal direction. The small and large comparing unit carries out a small and large comparison with respect to the horizontal accumulated value by each separated pixel number k to decide whether or not the reference pixel data is pixel data which is located in a repetitive object including a certain repetitive pattern.