PURPOSE: To increase the effective transmission capacity by using a frame synchronizing pattern consisting of less bit number after the frame synchronization is established.

CONSTITUTION: Till frame synchronization is established between a signal A sent from an equipment A and a signal 4 received by an equipment B, a signal 4 resulting from the sets of a basic frame synchronizing pattern 51 and a frame formed in a user data area 6 is sent from the equipment A to the equipment B. After the equipment A establishes the frame synchronization of the signal 4 passing through a transmission line 3b, an answer bit 7 is read and when the synchronization is normal, the basic frame synchronizing pattern 51 of the signal 4 using the transmission line 3a is changed into a reduced frame synchronizing pattern 52. Thus, the user data area 6 is assigned more by the decrease in the bit number and the effective transmission capacity after the frame synchronization is established is increased.