A intrusion detection system includes a first sensor device having a first transmission circuit unit that transmits a modulation wave of a first transmission spectrum diffusion signal to a first transmission leakage transfer passage and a second sensor device having a second transmission circuit unit that transmits a modulation wave of a second transmission spectrum diffusion signal to a second transmission leakage transfer passage. The signal interval of the first transmission spectrum diffusion signal is the same as the signal interval of the second transmission spectrum diffusion signal. The first transmission spectrum diffusion signal and the second transmission spectrum diffusion signal are shifted in frequency so that the first transmission spectrum diffusion signal does not overlap with the second transmission spectrum diffusion signal.