PURPOSE: To prevent a call from being disconnected by substituting a communication control module (CCM) capable of substituting communication because of the generation of extra load in the CCM out of plural CCMs other than a calling CCM generated within a fixed time for a calling packet exchanging function.

CONSTITUTION: When the load of a CCM 21 is congested, the CCM 21 receiving a call request P1 transmitted from a call terminal 51 sends a substitution request signal S10 to a call storing module 30. The module 30 temporarily stores the call request in a queue 70, sets up a substitution queuing time T1 and waits until another CCM is turned to a substitution enabled state. When the load of a CCM 23 is reduced less than a previously determined substitution enabled reference value and turned to a substitution enabled state, the CCM 23 sends a substitution instruction request signal S11 to the module 30. Thus, a call is prevented from being disconnected.