PURPOSE: To attain the changeover control to a standby path satisfying a requirement with respect to connection quality notified by the user at a fault of a transmission line of a communication network.

CONSTITUTION: Cross connect devices 7, 8 implementing path changeover between a standby transmission line 12 and active transmission lines 11a, 11b accommodating plural paths handled as a unit of line bundle between transmission installation terminal stations or lines connecting exchanges 1-6 are used to decide a standby path allocation quantity satisfying that the connection quality among the exchanges 1-6 is a preset criterion for each line group among the exchanges 1-6 when a fault takes place in the current transmission lines 11a, 11b of a communication network. The active path of the faulty active transmission line is switched to the standby path being the standby transmission line 12 according to the decided standby path allocation.