An optical signal cutoff detection circuit comprising: a comparator to input via a coupling capacitor, an electrical signal (Tout) obtained by photoelectrically converting an optical signal (Pin) formed from a pulse train, and to output pulses having amplitudes not less than a reference value as a comparison output signal (Cout); and an analog holding circuit to charge a holding capacitor (Ch) with each pulse contained in the comparison output signal (Cout) and to remove a DC voltage obtained by charging via a discharging resistor (Rh), thereby generating a holding output signal (Hout) that changes in accordance with presence/absence of input of the optical signal (Pin), characterized in that the electrical signal (Tout) is formed from differential signals including an inverted signal (Tout-) and a noninverted signal (Tout+), and the comparator includes a plurality of differential amplification circuits (11B, 11D) connected in series, and out of the differential amplification circuits (11B, 11D), a front-side differential amplification circuit located on a front side to differentially amplify the inverted signal (Tout-) and the noninverted signal (Tout+) using two load resistors (R5, R6) corresponding to the inverted signal (Tout-) and the noninverted signal (Tout+), respectively, and having different resistance values, thereby configured to output an inverted signal (Fout-) and a noninverted signal (Fout+) while applying an offset voltage, which is determined by the two load resistors (R5, R6) and serves as the reference value, between a DC bias of the inverted signal (Tout-) and a DC bias of the noninverted signal (Tout+), and a rear-side differential amplification circuit located on a rear side of the front-side differential amplification circuit, to differentially amplify pulses having amplitudes not less than the reference value out of the pulses contained in the inverted signal (Fout-) and the noninverted signal (Fout+) output from the front-side differential amplification circuit, and to output differentially amplified signals as the comparison output signal (Cout). a comparator to input via a coupling capacitor, an electrical signal (Tout) obtained by photoelectrically converting an optical signal (Pin) formed from a pulse train, and to output pulses having amplitudes not less than a reference value as a comparison output signal (Cout); and an analog holding circuit to charge a holding capacitor (Ch) with each pulse contained in the comparison output signal (Cout) and to remove a DC voltage obtained by charging via a discharging resistor (Rh), thereby generating a holding output signal (Hout) that changes in accordance with presence/absence of input of the optical signal (Pin), characterized in that the electrical signal (Tout) is formed from differential signals including an inverted signal (Tout-) and a noninverted signal (Tout+), and the comparator includes a plurality of differential amplification circuits (11B, 11D) connected in series, and out of the differential amplification circuits (11B, 11D), a front-side differential amplification circuit located on a front side to differentially amplify the inverted signal (Tout-) and the noninverted signal (Tout+) using two load resistors (R5, R6) corresponding to the inverted signal (Tout-) and the noninverted signal (Tout+), respectively, and having different resistance values, thereby configured to output an inverted signal (Fout-) and a noninverted signal (Fout+) while applying an offset voltage, which is determined by the two load resistors (R5, R6) and serves as the reference value, between a DC bias of the inverted signal (Tout-) and a DC bias of the noninverted signal (Tout+), and a rear-side differential amplification circuit located on a rear side of the front-side differential amplification circuit, to differentially amplify pulses having amplitudes not less than the reference value out of the pulses contained in the inverted signal (Fout-) and the noninverted signal (Fout+) output from the front-side differential amplification circuit, and to output differentially amplified signals as the comparison output signal (Cout).