The PD converts the light into a current signal and supplies the converted a current signal to a TIA and a light intensity measuring unit. The TIA converts the current signal into a voltage signal. The CDR circuit identifies whether the voltage signal is 1 data or 0 data for reproduction. The counter counts the 1 data and 0 data, calculates their ratio. The control unit refers to light intensity data from the light intensity measuring unit and a ROM, acquires an optimum ratio, and determines whether the ratio supplied from the counter is the optimum ratio. When the ratio is not the optimum one, the control unit controls the threshold voltage setting unit to set the threshold voltage so that the ratio is the optimum one.