Abstract: A discrimination circuit discriminates a signal with high stability without being affected by a waveform of the signal to be tested by using an optimum threshold level. The discrimination circuit includes, mark detection means 30 for measuring a mark rate of the signal 100 to be tested for a unit time T0 by applying a threshold level voltage which has a wider voltage range than said signal 100 to a comparator 42 from a DA converter 12, and discrimination control means 20 for specifying an average low level voltage and an average high level voltage based on two points that show abrupt changes in the mark rate in a mark rate distribution obtained by the mark detection means and for calculating a threshold voltage from both of the average low level voltage and the average high level voltage for controlling the DA converter 12 to set the threshold voltage.