An optical receiving apparatus includes a signal brancher for branching an optical input signal received from an optical transmission line into a first signal component and a second signal component, a clock extractor for extracting a clock having an amplitude from the second signal component, and a discriminator for discriminating the first signal component. The apparatus also includes a threshold controller for generating a discrimination threshold for the discriminator according to a relationship between the extracted clock amplitude and a bit error rate to which the extracted clock amplitude corresponds.