Abstract: An optical receiver includes a soft-decision deciding means (7) for deciding an electric received signal according to a plurality of decision levels so as to output a multivalued decision signal, a demultiplexing means (5) for serial-to-parallel converting the multivalued decision signal so as to output a multivalued parallel signal, a soft-decision error correction decoding means (8) for making an error correction to the multivalued parallel signal based on reliability information so as to output an error-corrected parallel received signal and decision results indicating the decision of the electric received signal according to the plurality of decision levels, a probability density distribution estimation means (9) for estimating probability density distributions based on distributions of the decision results indicating the decision of the electric received signal according to the plurality of decision levels, and a decision level control means (10) for controlling the plurality of decision levels in the so