According to a decision threshold voltage controlling method of feeding an optimum decision threshold voltage to a clock and data recovery circuit which converts an optical input signal into an electric signal, extracts a clock component from an input data signal amplified to a predetermined amplitude, and identifies 1 or 0 in the input data signal at a timing of the clock, three decision points are suitably controlled, by selectively performing a process of controlling the spaces of the three decision points, a process of moving the three decision points with their spaces kept as they are, and a process of changing an error pulse measurement time, depending on a measurement result of the error pulse.