PURPOSE: To attain supervision of high transmission quality by performing a reception state signal transmission of a high-speed digital signal of an opposite device and pseudo random code transmission of bit error rate estimation of the high-speed digital signal with one signal to improve the utilizing efficiency of the bit error rate estimation.

CONSTITUTION: The high-speed digital signal dh' received by a supervising circuit 107 of a reception signal is supervised always and 1 or 0 is added to an AND gate 201 depending on the presence of the signal dh'. Further, a pseudo random code prs is generated from a pseudo random code generator 101 and a logical product is outputted from the gate 201. An output of the gate 201 and low-speed digital signal from terminals 102, 103 are outputted while being subjected to time division multiplex at a multiplex circuit 104. Moreover, a separation circuit 108 separates the signal dh' into a low-speed digital signal, an erroneous bit of the pseudo random code is counted by an erroneous bit counter circuit 109 and a discriminating circuit 202 discriminates the state of the signal. Then the utilizing efficiency of the high speed digital signal is increased.