Abstract: A method for detecting a fine frequency deviation generated in a transmission line such as a general public telephone switching network. The method includes the steps of transmitting test signals to a transmission channel to be tested, receiving the test signals from the transmission channel, detecting a basic period from the received test signals, and determining a measure period on the basis of the detected basic period, performing a normalization by use of the basic period determined, extracting values of signals to be calculated, namely, object signals from those of the basic and measure periods, calculating respective absolute values of the object signals, averaging the calculated absolute values, and thereby calculating an absolute value of frequency deviation and determining a sign or direction of the frequency deviation, and determining a final frequency deviation on the basis of the sign determined from the absolute value of frequency deviation.