PURPOSE: To remove automatically a higher harmonic, a distortion component, by providing a tone-quality change circuit which attenuates the high-band component of a low frequency signal demodulated by the output of a multi-path detection circuit, besides the multi-path detecting circuit.

CONSTITUTION: When no multi-path occurs, a higher harmonic output is not derived to HPF82 of multi-path detection circuit 8 since the output of detection circuit 5 contains no higher harmonic component, so that the output of Schmitt circuit 83 becomes low in level. As a result, transistor 92 of tone-quality change circuit 9 becomes unconductive and no high-band attenuation is caused by LPF of resistances 93 and 94, and capacitors 95 and 96. When the output of detection circuit 5 contains the higher harmonic component due to the occurrence of a multi-path, a positive voltage is developed at the output of Schmitt circuit 83, allowing transistor 92 to conduct. Therefore, the high-band component in detection circuit 5 is attunuated by LPF of resistances 93 and 94, and capacitors 95 and 96, thereby automatically removing tone distortion.