To provide a system for determining a fault block on a digital line from an input signal and an output signal of one device arranged on the line in the case of line fault without affecting communication between users at an ordinary time.

In the fault block determination system, a scrambler and a descrambler are arranged on the relevant line, the scrambler has a polynomial for scrambling a series of "0" and a series of "1" to any series other than "1" independently of a status of a shift register and when an input signal of the descrambler is a series of "1", it is determined that a fault block exists between the scrambler and the descrambler. When an output signal of the descrambler is a series of "1", it is determined that a fault block exists far from the scrambler.