PURPOSE: To attain data communication with high quality and high speed by setting a synchronous character number at every communication path.

CONSTITUTION: The communication path from an electronic computer 1 to an electronic computer 7 is set as exchange 2→3→6 or exchange 2→4→5→6, and when the communication path is set, a synchronous character number control mechanism 9 in the electronic computer 1 sends synchronous characters by the number in matching with the communication path. The synchronous character number is the sum of a difference between the synchronous character number sent as the test by the electronic computer 1 and the synchronous character number arrived at the electronic computer 7 via the set communication path and the synchronous character number required for the electronic computer 7. Thus, the synchronous character number arrived via the communication path being the multi-stage connection of the exchanges 2-6 is detected to decide the quality of the communication path and the synchronous character number is decided at every communication path to attain stable data communication with high speed and high quality.