LPF and HPF extract bass and treble ranges, respectively, from an input sound signal, and bass and treble boost circuits perform dynamic range expansion/contraction on the extracted bass- and treble-range sound signals in accordance with input levels of the sound signals. The input sound signal and the sound signals output from the boost circuits are added together. There may also be provided coefficient calculation sections for calculating filter coefficients on the basis of the levels of the sound signals extracted by the LPF and HPF. In this case, the bass and treble boost sections perform, in accordance with the filter coefficients calculated by the corresponding coefficient calculation sections, filter processes for increasing/decreasing the levels of the bass and treble ranges, respectively.