PURPOSE: To prevent the generation of noises owing to the hopping of frequency by changing the transmission state of the spread spectrum signal in response to the state of the information signal and therefore detecting the information signal just through detection of the state of the spread spectrum signal.

CONSTITUTION: The output of an AND circuit 1a is kept at level 0 when the information signal of level 0 is supplied to a terminal 7. While a clock signal is transmitted to a terminal 41a of a switch 41 when the information signal of level 1 is supplied to the terminal 7. Therefore a frequency converter 40 transmits the signal supplied from a synthesizer 3a as it is when the information signal is set at a level 0. While the converter 40 transmits a signal of a sum frequency of the signal supplied from the synthesizer 3a and the signal supplied from an oscillator 42 when the information signal is set at a level 1. As a result, the signals of different frequencies are delivered through an antenna 5 every half cycle of the clock signal.