An optical receiver for stably reproducing packets having different light receiving levels is disclosed. The optical receiver includes: a light receiving element for outputting a current in response to a light receiving level of an optical signal; a preamplifier for converting the current signal outputted from the light receiving element into a voltage signal; a circuit for detecting a consecutive same binary symbols portion from a binary symbols stream of the voltage signal outputted from the preamplifier to output a time constant switching signal in response to a detection result thereof; a level detecting circuit for detecting a voltage level of the voltage signal outputted from the preamplifier based upon a time constant which is switched/controlled in response to the time constant switching signal; and an amplifier for amplifying an output voltage of the level detecting circuit to apply a control voltage for controlling the gain to the preamplifier.