To provide a detector capable of reducing misrecognitions for an intrusion/obstruction, and capable of attaining low electric power consumption.

This detector has a sensing unit and a responsor, the responsor has a filter for filtering frequencies other than a specified frequency out of the radio wave frequencies received from the sensing unit, and an antenna for transmitting the specified frequency of radio wave transmitted through the filter, to the sensing unit, without amplifying it. The sensing unit has a transmission means for transmitting the radio waves of the plurality of frequencies, including the frequency serving as a reference and the specified frequency from the antenna, to the responsor, and a deciding means for deciding the presence of intrusion or obstruction, based on the presence of the reception of a specified frequency of radio wave and the presence of the reception of the radio waves of the frequency serving as the reference.