A monitor for intrusion detection features a detecting arrangement for detecting an intruding object in a 3-dimensional area and outputting corresponding detection information, a setting arrangement for setting information necessary for monitoring the position or actions of the intruding object in the 3-dimensional area, a monitoring information generating arrangement for generating monitoring information concerning the position or actions of the intruding object in the 3-dimensional area, on the basis of the detection information generated by the detecting arrangement and the setting information by the setting arrangement, and an external output arrangement for outputting an control output or a display output according to the monitoring information concerning the position or actions of the intruding object generated by the monitoring information generating means.