A clock data recovering circuit solving a problem in which a stable clock signal cannot be extracted is provided. A phase comparator includes a main-signal-discriminator. The main-signal-discriminator discriminates a reception signal by a clock signal to generate recovery data indicating the discrimination result. Phase comparator 2 uses the discrimination result of the main-signal-discriminator to compare phases of a reception signal and a recovery clock and outputs a phase comparison signal indicating the comparison result. A generator generates a recovery clock with a frequency corresponding to the comparison result indicated by the phase comparison signal outputted from phase comparator 2. An eye opening monitor detects an optimal discrimination point of main-signal-discriminator 1 based on a monitor signal split from the reception signal and the recovery data generated by the main-signal-discriminator. The eye opening monitor also adjusts a discrimination point of the main-signal-discriminator based on the detection result.