To prevent the side lock of AFC even when the precision of TCXO (temp. compensation-type crystal oscillator) is deteriorated in the AFC control of a mobile body communication system which is represented by a PDC(personal digital cellular) system digital portable telephone.

An AFC side lock preventing circuit is provided with a TCXO 10 to be an original vibration of an RF part and a voltage control oscillator(VCO), the RF part 11 of for converting a signal received from an antenna down to an IF band, the VCO 12 for generating an IF×N clock, a frequency difference detecting part 13 for detecting a frequency difference from an IF signal, a frequency correcting part 14 for correcting a frequency from a frequency correction pulse and outputting an after correction IF signal (AFCIF), an AFCIF counter 15 for counting the AFCIF signal for a fixed period, an IF counter 18 for counting the IF signal for a fixed period and a CPU 16 for controlling a D/A converter 17 which varies a voltage to TCXO 10 by the measurement results of the AFCIF counter 15 and the IF counter 18. Thus, the side lock of AFC is prevented even when TCXO 10 is deteriorated in precision.