PURPOSE: To eliminate a large capacity storage area and to convert data at real time by installing a data form conversion device which stores image data, by a specified bit number, slantingly in sequence by every storage unit of mem ory and reads out the data in order of the column or line of a matrix.

CONSTITUTION: A printing start signal 16 is input into MPU 1 from an image controller, a controller which stores image data, etc. MPU 1 outputs a printing action start signal to a data form conversion part 2 and a printing control part 3. Additionally, MPU 1 generates a drive signal to transfer a printing part transfer part 6. The data form conversion part 2 divides image data P (slice data) by M bits input from an image data line 7 to a register 2a of M bits × K (K=M/L) by L bits and stores slantingly. In outputting to a printing data line 11, data of M bits which are lined up longitudinally are output for the conversion of data.