Abstract: A hearing aid tailored to a hard-of-hearing person is designed using sampled-data control theory. The method is for designing an audio signal processing system for a hearing aid, wherein the system comprises an AD converter for converting an analog audio input signal (yc) inputted to the hearing aid into a digital audio input signal, a hearing aid digital filter (K(z)) for performing a signal processing on the digital audio input signal outputted from the AD converter, and a DA converter for converting a digital signal outputted from the hearing aid digital filter into an analog audio output signal to be outputted to the hard-of-hearing person.