To provide a monitoring method for distinguishing a mobile unit, such as an authorized human being from an unauthorized intruding object.

A transmitter 10 transmits radio waves 1 having a specific pulse signal into a monitored region 18. The radio waves, and radio waves, obtained by the reflection of the radio waves within the monitored region 18, are received by an antenna 16, thus detecting the electric field strength characteristics value in the radio waves at an electric field strength characteristics value detection section 22. Then, when a variation detection section 23 detects the variation in the electric field strength characteristics value, a verification radio wave transmission section 25 transmits radio waves for verification. After that, a response radio wave reception section 26 receives radio waves for response from a terminal apparatus 80. A reporting section 27 determines whether an object that becomes a factor for the variation is valid, based on the presence or absence of the reception of radio waves for application.