To realize printing data communication at faster speed without selecting an interface by a user in a printing data processing system having two or more communication interfaces.

When a printing demand is generated from a host computer, a throughput of the whole line is measured and an interface communicable with the most high-speed data is selected out of the interfaces, and the printing data are transmitted to the printer by using the selected interface. When there is a line in which the completion of throughput measurement should be delayed because of a break in the line and the like in the measurement of the throughput of the line; the line is not used even though the line is available for high-speed data communication, and the most high-speed line is selected out of only the lines, of which the throughput measurement is completed, When the printer can receive the data from two or more interfaces, the printing data are divided and the printing data are transmitted by the two or more interfaces based on the throughput measurement, while taking the bandwidth covered by each line into consideration.