A low-pass filter in a read channel, having an adjustable cutoff frequency has a low-pass filter 14, a time measuring circuit 15, a storage computing circuit 19 and a current supply circuit 18. The time measuring circuit 15 computes the pulse number of a reference clocking signal. The storage computing circuit 19 obtains the mean value of first and second set values that correspond to current values of the control current when the pulse number of the reference clocking signal increases by 1, and the current supply circuit 18 supplies a control current equivalent to the mean value to the low-pass filter 14. Therefore, even if the same pulse number is computed more than once, the desired current set value can be obtained from the mean value of the first and the second set values, and a control current amount that corresponds to said current set value can be supplied. Consequently, even if the frequency of the reference clocking signal is low, the cutoff frequency can be finely set by finely setting the value of the control current.