A carrier-to-noise detector comprises an A/D converter (1) which samples an output of the demodulator of a digital transmission system at a symbol clock rate and converting it to a digital signal having positive and negative values. An absolute value converting circuit (2) converts the output of A/D converter into an absolute value which is averaged by a first averaging circuit (3) over a period sufficient to suppress short term variations and then squared by a first squaring circuit (4) to give an output representing the carrier component. The output of A/D converter is, on the other hand, squared by a second squaring circuit (5) and averaged by a second averaging circuit (6) to suppress short term variations to give an output representing a total of the carrier and noise components. The carrier component in the output of the second averaging circuit 6 is subtracted by a subtractor (7). A ratio between the outputs of the first squaring circuit (4) and subtracting circuit (7) is derived by a divider (8) as a representation of a carrier-to-noise ratio.