PURPOSE: To reduce retrial frequency of bypassing and to eliminate the need for a common memory by shifting a bypass destination unit each time a bypass- call is generated when an exchange unit has no idle line and bypassing between units is required.

CONSTITUTION: When a calling is generated at a calling-generation exchange unit 1 and at that time an idle line exists for a path in the destination direction in the unit 1, connection is made with an output highway 7-1 contained in the calling-generation exchange unit 1 by way of a path 9-2. If there is no idle line for a path in the destination direction in the unit 1, based upon an idle channel information 3 transferred between respective exchange units, a bypass destination exchange unit 2 which has an idle line and exists in the destination direction is obtained at the unit 1. Then, a calling is connected to an output highway 7-2 contained in the unit 2 by way of (a unit connection highway 8-1, a path 9-3 in an inter-unit switch 4, a unit connection highway 8-2 and a path 9-4 in the unit 2).