A semiconductor device includes a first trans-impedance amplifier, a second trans-impedance amplifier, a peak hold circuit, a comparator and a threshold current setting circuit. The first trans-impedance amplifier converts a first current signal generated by a first photodiode, into which an optical signal is input, into a first voltage signal. The second trans-impedance amplifier converts a second current signal generated by a second photodiode, to which an optical signal is blocked, into a second voltage signal. The peak hold circuit holds the peak value of the first voltage signal. The comparator outputs a pulse on the basis of the first and second voltage signals. The threshold current setting circuit draws out a threshold current.