An equalizer adjusting device includes a comparator, an inequality counter, an adjuster, and the like. The comparator performs magnitude comparison between a voltage value Vout of each bit output from an equalizer and a threshold value MonLVL and outputs a logical value MonSMP according to a result of the comparison. The inequality counter inputs a logical value DatSMP output from a sampler in accordance with the result of magnitude comparison between the voltage value Vout of each bit and a reference value, and the logical value MonSMP output from the comparator and counts events in which the logical value DatSMP and the logical value MonSMP differ from each other, every period. The adjuster adjusts a gain of the equalizer and the threshold value MonLVL of the comparator based on a counted value of the inequality counter.