To provide a signal detecting circuit which has a high allowance degree against noise, an excellent receiving sensitivity and large proof press against impulsive noise.

When an analog INFO1 signal is inputted, it is rectified into double voltage of conventional receiving voltage Vinfo1 by a voltage doubler rectifier circuit 80, and a reference voltage setting circuit 90 inputs comparator input voltage Vdef=Vin+-Vin- of V1+V2=2×Vinfo1 to a comparator 100. Therefore, an output signal of the comparator 100 becomes 'H' and a loop switch in a digital line terminating device is turned on. When impulsive noise that has high frequency components in comparison with the analog INFO1 signal is inputted, the high frequency components are eliminated by a lowpass filter that consists of resistance 70 and capacitors 83 and 84.