Brake squeal is uncomfortable noise that occurs while braking. So, it is an important issue of automobile quality to reduce the brake squeal occurrence. Although many researchers had reported about brake squeal, there are many examples that have not been understood well.For those squeals, some of occurrence mechanisms are assumed. The dµ/dv<0 theory is one of them, and the dµ/dv represents a slope of friction coefficient for sliding velocity. If the frictional force has a negative damping characteristic, that is dµ/dv<0, disc vibration may increase. It has been said for a long time that a negative dµ/dv characteristic affects brake squeal occurrence.Many researchers had measured the dµ/dv to clarify brake squeal factors. In those results, the slope of friction coefficient at steady sliding speed was used for the dµ/dv. However, the pad undergoes minute vibrations while the brake is squealing. So, we developed a device that can measure the friction coefficient with minute vibrating velocity. A piezo-actuator and a dynamic load sensor are used to measure the friction coefficient at minute vibrating velocity.The results say that dµ/dv at the minute vibrating velocity differs from that at the steady sliding speed. And, the results also showed that dµ/dv at the minute vibrating velocity tended to increase for actuating frequency, and tended to slightly decrease for thrust load.