In a magnetic disk device, a deterioration of performance caused by an environmental variation is suppressed by utilizing an automatic adaptation function in a read/write channel IC. A gate provides a timing for the automatic adaptation of an equalization coefficient, a signal from the gate is given to a read-gate input terminal of the read/write channel IC, and the gate for the automatic adaptation is opened before reading data from a destination sector to be read. The automatic adaptation is started, whereby coefficients for equalization are adapted to signal wave forms before reading data.