PURPOSE: To attain miniaturization and to reduce the cost by incorporating an operational amplifier to a sender side output stage and a receiver side input stage of a ciphering IC and providing an external connection terminal to the IC.

CONSTITUTION: A voice signal 100 inputted from an audio amplifier 3 to a ciphering IC40 is subject to impedance conversion by an input buffer 12. Then frequency inversion modulation is implemented, a high frequency component is cut off and the result is outputted. The output signal is subject to band correction such as pre-emphasis by a circuit comprising an operational amplifier 42, a C155, a R156, and a R157. The signal subject to band correction is sent to an opposite radio receiver via an antenna 8 by a transmitter 7. Thus, the band correction circuit and the amplifier circuit keeping stable characteristic are formed by using only passive components such as capacitors and resistors.