To quickly generate appearance frequencies (histogram) of density values of image data.

Input data is delayed with a D-FF 110 by one cycle, and delay data and data which is not delayed by the D-FF 110 are alternately selected and outputted in every cycle by a second selector 112. Data from the second selector 112 is taken as an address to read data from a memory 116 in the first half of a cycle, and data resulting from adding one by an adder 120 is written to the memory 116 in the latter half. A comparator 108 discriminates coincidence or non-coincidence between two consecutive data, and data changed by a changer 102 is selected by a first selector 104 and is supplied to a D-FF 106 in the case of coincidence.