PURPOSE: To decrease a shock to ears and to prevent an unexpected accident by applying waveform shaping to a ring frequency signal so that the leading of a ring tone is unsharpened and the tone volume is increased gradually.

CONSTITUTION: When a call via a telephone line exists and a ring frequency signal Sr generated based thereupon is detectes by a ring frequency signal detecting circuit, a ring detection signal Sd rising to an L level is generated. When the signal Sd goes to the L level, one terminal of a resistor R0 participating in the forming of time constant in a time constant circuit 11 is grounded substantially and the discharge of a capacitor C0 follows the time constant (τ=R0.C0). Thus, a base of a transistor (Tr)4 falls down gradually toward ground and the main current carrier of the Tr4 based on the ring frequency signal Sr is increased gradually. Then a signal waveform given to the input of an audible band amplifier 1 becomes a ring frequency waveform signal Srm after waveform shaping and the tone volume is increased gradually.