PURPOSE: To obtain an optimal transmission rate by decreasing and adjusting the transmission rate of data to a printer so that the data from the printer and the data generated by a printing data generating device coincide with each other.

CONSTITUTION: A command of a transmission rate optimizing instruction and test data generated by a data generating part 12 are transmitted to a printer 2. Subsequently, data transmitted from the printer 2 by the command of the transmission rate optimizing instruction and the test data generated by the data generating part 12 are compared by a comparing part 14, and unless both the data coincide with each other, a wait time is inserted into a strobe signal transmitted from a parallel port part 13 to the printer 2 and a data signal, and the processing is repeated until those two data coincide with each other by the comparing part 14. Also, when they do not coincide with each other even if it is repeated by a prescribed number of times, by changing a transmission system synchronized with both edges of the strode signal to a transmission system synchronized with one edge (fall), the previous processing is repeated again.