PURPOSE: To obtain a variable equalizer where the intercode interference in the pulse transmission is small and the S/N change due to noise components and eye degradation components is minimized, by cascading a 5-order maximum delay flat filter and a specific emitter peaking amplifier.

CONSTITUTION: A 5-order maximum delay flat filter 24 and an emitter peaking amplifier 25 which resonates near the frequency, where the gain is lowered by 3dB, in the high band of the frequency characteristic of this filter 24 are cascaded. The resonance frequency of the amplifier 25 is changed to correct the change of the frequency characteristic in the optical fiber transmission. In this equalizer, it is desirable that the ratio (fc/fo) of a cutoff frequency fc of the overall frequency characteristic between the filter 24 and the amplifier 25 to a pulse repeat frequency fo is 0.18∼0.3 (optimum 0.25).