To extract data in a random data area used by a user correctly as digital binary data string by deciding a reference value obtained by integrating a demodulated base band analog signal within a preamble area.

A base band analog BA signal extracted by a radio demodulation part 2 is transmitted to a comparator 7 and a switch 6. The switch 6 is closed and opened by H/L of a slice control signal from a control part 3 and at the time of closing, the BA signal is integrated by a resistor 10 and a capacitor 11 and appears at the negative side measuring point 9 of the comparator 7. When the switch 6 is in a closed state, the hold voltage of a voltage value at the time point when the slice control signal is switched from H to L appears at the positive side measuring point 8 of the comparator 7. The comparator 7 compares the BA signal and the hold voltage, and 1 is outputted to the comparator 7 when the BA signal is high but 0 is outputted when the BA signal is low to make a binary data string to send to the control part 3 through a transmission line 13.