1. A disk storage device for detecting data from a reproduced signal read from a disk and decoding the data, the device comprising: an A/D converter for sampling a waveform of the reproduced signal at a predetermined clock cycle and converting the reproduced signal into a digital signal; a linear equalizer for carrying out a linear waveform equalizing operation on the digital signal converted by the A/D converter in order to reduce a mean square error between the digital signal and an amplitude of an equalization target waveform; and a nonlinear equalizer for carrying out a feedforward neural network type nonlinear waveform equalizing operation on sampled values of a waveform of the digital signal linearly equalized by the linear equalizer.