A radio receiver receives a 4-FSK signal through a radio system (101) and a detector (102) detects a four-level detected signal from the 4-FSK signal. The four-level detected signal is converted into digital form by an AD converter (104). The digital signal is stored onto a memory (114). The AD conversion is performed at intervals obtained from the four-level detected signal. A CPU (112) performs the data processing of the received digital data when the AD conversion is not performed and does not perform it when the AD conversion is performed.