PURPOSE: To perform an optimum correction even when a difference is present between the front half position and the rear half position of an undershoot in the undershoot correction in a magnetic storage device using a thin film head.

CONSTITUTION: A delay circuit A1 delays a read-out signal before equalization A100 being an input signal by a prescribed time and reflected waves from outputs are present in respective signals 102 to 107. Next, a signal after slimming equalization A109 is obtained by excuting the subtraction between a slimming signal B108 and a read-out signal before equalization B101. Then, the correction of the undershoot is performed by adding an undershoot correction signal F114 selected from undershoot correcting signals A103, B104..., E107 and the signal after slimming equalization A109, however, the optimum correction can not be performed since the deviation of a balance is generatred when the difference is present between positions of the front and the rear of the undershoot. For that reason, the balance is adjusted in accordance with a large difference between the front half position Tf and the rear half position Tr of the undershoot with a delay circuit C3 and a delay circuit B2 so that the correction becomes optimum.