PURPOSE: To shorten a time required until a receiver synchronizes with a reception signal to the utmost in the radio communication in which a continuous signal is transmitted.

CONSTITUTION: This system is provided with a C/N discriminator 3 and a frequency step controller 4, the C/N discriminator 3 discriminates the reception state of a received IF signal and a frequency step at sweeping is changed based on the C/N. This, when the C/N is large, the frequency step is increased to allow the receiver to synchronize with a reception signal in a shorter time, and when the C/N is small, the frequency step is decreased to allow the receiver not to cause synchronizing signal detection mistake.