A data processing apparatus comprises: a memory for storing image information as bit image data; a data converter for overlapping the bit image data with another data in the memory; and a control unit having a microprocessor for instructing the operation of the data converter, in which the data converter is activated by an activating order and performs the overlapping operation while the control unit is executing another order. The data converter has a data shifting circuit to shift the foregoing data by only desired bits and overlap it on the bit image data. The overlapping operation of the data into the memory and the clearing operation of the memory can be synchronously performed. With this apparatus, a memory having a large capacity can be effectively used and the burden on the CPU can be reduced upon writing of the data into the memory and a high data processing speed is obtained.