Detection device and detection system

A detection system, comprising:

• a reader that transmits and receives a radio wave including a signal and transmits a transmission signal including at least a power supply signal;
• a detection device including:
• an antenna that transmits and receives a signal to and from the reader;
• a signal output unit including a circuit terminal connected with the antenna, the signal output unit being activated with power derived from the power supply signal included in the transmission signal received by the antenna to output a detection signal to the antenna;
• and
• a gain reduction unit that switches, according to a state of an object to be detected, between a gain reduced state in which a gain of the antenna is reduced and a gain unreduced state in which the gain of the antenna is not reduced;
• and
• a determiner that is connected with the reader and determines a state of the objected to be detected with the detection signal received by the reader, wherein
• the gain reduction unit has a point of action for reducing the gain of the antenna, when λ is a wavelength of the radio wave, the point of action is located within a gain reduction range that represents a range on the antenna and that extends from the circuit terminal to a position of λ/32 away from the circuit terminal along a direction in which the antenna extends.