Abstract: The invention is a method and a circuit for detecting the carrier-to-noise ("CN") ratio of a quaterary phase-shift keying ("QPSK") signal. A binary signal including in the QPSK signal is reproduced by a converting circuit including a QPSK demodulator and a difference converter. The binary signal is then error-controlled by an error detecting circuit and the detection of a double-error. The occurrence number of the double-error is counted during a predetermined time period which is long enough to compare with the occurrence time when two successive double-errors occur. A microcomputer stores a conversion table corresponding to the theoretical relation between the CN ratio and the occurrence number. The microcomputer computes the CN ratio from the detected occurrence number. The theoretical relation is formulated based on the assumption that noise distributes isotopically around the QPSK signal in the signal space in accordance with the nomal distribution.