PURPOSE: To detect a change between an initial timing extraction pattern and a training pattern when the difference is a prescribed value or over and to prevent mis- detection by comparing power between 2 signal points with a distance when a rate of change in the distance is at a prescribed value or over.

CONSTITUTION: Since a line signal at points A, B is sent alternately during a segment 2, even when distortion of a transmission line is caused, the points A, B are converged to respective points as shown in figure. Thus, the distance between the coordinate of the signal received at a time and that of the signal received at two preceding time is almost zero in the segment 2. When a signal of a point C is sent after the changeover to a segment 3, the distance takes a large value suddenly and since a signal of points C, D is sent at random in the segment 3, they are not converged to one point and the distance between the point received at a time and the signal received at two preceding time is a large value. Since the signals of the two points having the distance come from the same signal point, the power is the same. When the distance between the signal points takes a large value suddenly and the difference from the power of the 2 signal points is taken, and when the value is larger than a prescribed value, the detection processing is continued and when smaller, it is discriminated to be at the switching state.