A detecting system includes: a sensor (10) that includes an antenna unit (11) formed with a metal pattern, and a back surface reflector (13) that faces the antenna unit (11) via an isolation layer (12); and a reader (20) that transmits electromagnetic waves (Fa) to the sensor (10), receives reflected waves (Fr) from the sensor (10), and compares the reflection characteristics of the sensor (10) detected from the reflected waves (Fr) with the reflection characteristics of the sensor (10) stored in advance, to detect a state change in the sensor (10).