A bit rate control apparatus having a delay unit for delaying an input signal received in an optical receiver in an optoelectrically-converted state; a DC level outputting unit for exclusively OR'ing the input signal with the delayed signal outputted from the delay unit; an A/D converting unit for A/D converting a DC level outputted from the DC level outputting unit; a clock/data reproducing unit for reproducing clocks and data based on a bit rate control signal; and a control unit for calculating a variation in the DC level at every interruption timing, which is previously set, based on the A/D converted signal, and for determining whether the DC level variation occurs under the influence of temperature or due to a variation in bit rate based on a difference between the calculated DC level variation from a predetermined variation limit.