The prevention of brake squeal in disc brakes is an important concern in designing a brake system because, in terms of quietness, brake squeal reduces a car's commercial value. Over the past years, many studies have been conducted to elucidate the mechanism underlying the occurrence of brake squeal. However, since disc brake squeal is a complex issue caused by the compound influence of friction phenomena and vibrations in the brake system, the problem of brake squeal still has not been completely resolved, even today. In order to propose an effective measure for the prevention of brake squeal, it is necessary to understand the nature of the brake squeal phenomenon.We have investigated the influence of the dynamic stiffness of a brake pad (hereinafter referred to as pad stiffness). The pad stiffness was evaluated the results by a pad excitation at frequencies and vibration amplitudes that would cause brake squeal. As a result, one of the factors such as the pressure dependence of the pad stiffness has been known to be a casual factor in the occurrence of brake squeal. Based on these findings, braking systems that minimize brake squeal are developed in the initial design phase. However, even if a brand-new brake pad does not squeal, the wear of the pad may eventually cause brake squeal. The brake structure does not change during braking, but the condition of the pads does because of sliding with the disc rotor. In order to design brakes that do not squeal over the long-term, it is necessary to identify the relationship between brake squeal and the change to pads as they wear down under the force of sliding with the disc rotor.This study set out to identify the effects that pad thickness and surface roughness have on pad stiffness as the pad wear down. First, we measured the pad stiffness for pads of different thickness. The results showed the pad stiffness increased as pad thickness decreased. Second, we measured the pad stiffness for pads of different roughness. The results showed the pad stiffness increased as pad surface roughness became smoother, especially on the lower pressure range. Finally, base on measurement results, the pad stiffness could be expressed as a series of spring representing a matrix stiffness caused by pad thickness and a surface stiffness caused by pad surface roughness. We examined that the influence of matrix stiffness and surface stiffness had on the pressure dependence of pad stiffness. The results showed that a matrix stiffness was little dependent on pressure, therefore it dominated pad stiffness on the higher pressure range on the one hand, surface stiffness was highly dependent on pressure, therefore it dominated pad stiffness on the lower pressure range.