PURPOSE: To prevent the deterioration of transmission quality by informing a specific bit code position to be bit-stealed in accordance with link connection and delaying the signal so that the specific bit code position coincides with a specific bit code position to be connected.

CONSTITUTION: A reception transmitting line 110 is connected to a receiving part 611 in a digital switchboard 700 and the receiving part 611 is connected to a switch network 300 through 24 receiving service channels. On the other hand, a transmission transmitting line 213 is connected to a transmitting part 212 in the switchboard 700 and the transmitting part 212 is connected to the network 300 through 24 transmitting service channels. It is supposed that a receiving service channel is linked with a transmitting service channel. A delay sending part 613 formed in the receiving part 611 receives time position information for a reference signal from the transmission service channel and a transmission service channel number from a connection controlling part 800. Consequently, the receiving service channel is delayed and a specific receiving frame to be bit-stolen is made coincide with the specific transmitting frame.