PURPOSE: To diagnose only a digital highway by commanding the connection of incoming and outgoing highway channels to a digital switch, sending a test data, collating the data with a data extracted from a designated channel of an outgoing highway so as to confirm the normal state of the equipment.

CONSTITUTION: When a designated channel is connected by a digital switch 6, a 1st channel system equipment 1 and a 2nd channel system equipment 3 send a test data to a channel of relevant incoming highways 2a, 4a. Then a data extracted from a channel of outgoing highways 2b, 4b and a data sent to an incoming channel are collated to confirm the normal state of the highways. Thus, only the digital highway is diagnosed.