To provide a monitoring system and a monitoring method capable of suppressing monitoring errors, even when there are many electric waves having frequency bands that are the same as or adjacent to that of an electric wave to be used, in a structure in which human activities are monitored in a predetermined area by using the electric wave.

A monitoring system 201 comprises: a transmitter 151, which is installed in a predetermined area, for transmitting an electric wave; and an invasion detection device 101, which is installed in a predetermined area, for receiving the electric wave transmitted from the transmitter 151 and monitoring human activities in the predetermined area on the basis of the received electric wave. The transmitter 151 repeats a transmission period of an electric wave and a transmission stop period. Then, the invasion detection device 101 calculates a space feature amount in a predetermined area by using an electric wave received within the transmission period of the received electric waves and monitors human activities in the predetermined area on the basis of the calculated space feature amount.