An adaptive cable equalizer includes a data signal input unit, a clock signal input unit, a variable equalizer that inputs a data signal input from the data signal input unit, and a transition time measuring portion that measures a transition time of a data signal output from the variable equalizer, with an equalizer control loop being configured that controls characteristics of the variable equalizer based on the output signal of the transition time measuring portion. The adaptive cable equalizer further includes a control circuit that controls response characteristics of the control loop according to the frequency of a clock signal input from the clock signal input unit. This enables a quick response at fast transfer rates by making the relationship between the response time of the control loop and the number of data bits substantially constant even when the transfer rate changes from a slow transfer rate to a fast transfer rate.