PURPOSE: To improve transfer efficiency by compressing parallel data having the same contents and then performing serial conversion when parallel-serial conversion is carried out.

CONSTITUTION: The 1st-byte data D0 of a parallel signal stored in a converting buffer 10 is stored 11 and data D1 is then stored 11; and both are compared 13 and a coincidence counter 14 counts up when they coincide with each other. Data D1∼D7 are compared 13 with the data D0 similarly, and when the data D1∼ D7 are all coincident with the data D0, a controller 21 allows a parallel-serial converter 20 to perform the serial conversion of a compressed display code Cc19, the data D0 as a reference, and quantity data N. When the data D1∼D7 are not coincident with the data D0, the serial conversion of the data D0∼D7 is performed by the parallel-serial converter 20 in sequence. The serial conversion of parallel data having the same contents is performed after compression to improve the transfer efficiency.