PURPOSE: To eliminate the change in a frame aligner even when the transmission speed is changed by making the start time point of a frame period, the number of sub frames and each sub frame the same with respect to all transmission speeds and arranging decentralizingly a communication field independently of the transmission speed to the head position of each sub frame.

CONSTITUTION: In the frame constitution at the transmission speed of 4.096, 2.56, 2.048 and 1.536MHz, a communication field independently of the transmission speed is arranged at a common position at the head of two sub fields and the content of a frame counter is made in common at the start point of time of each sub field. That is, in providing a 3-bit counter 5 dividing the transmission clock frequency into 1/8, a 4-bit counter 4 varying the frequency division ratio in response to the transmission speed, a 1-bit counter 3 and a 1-bit counter 2 providing the number of sub frames the count of the frame counter issuing the start of each communication field is made identical, and each communication field is assigned in common to each transmission speed.