external sensor 28 refers to a time when the value of such gauge is changed or the emergency button is depressed the abnormal time sensitiveness level switching means 7 includes the abnormal situation input section 71 the abnormal situation detection section 72 and an abnormal situation file holding section 74 an abnormal situation file held by the abnormal situation file holding section 74 is a file in which an abnormal situation and setup id of a sequence of levels of sensitiveness to received signals of the receiver which is applied when such abnormal situation occurs are recorded the u201c abnormal situation u201d refers to abnormal situations such as a case in which a walker present in the observation area 26 is not detected a case in which the emergency button installed in the observation area 26 is depressed or a case in which the gauges such as a voltmeter and a thermometer installed in the observation area indicate usually impossible values as shown in fig1 the abnormal situation file contains for example a sequence of abnormal situations es 1 1 u2266 l u2266 n n the number of registered abnormal situation sequences which is a sequence of numbers expressing values which indicate abnormal conditions of the receivers or the external sensors as elements and setup id unique to the sequence of levels of sensitiveness to received signals which is set when the abnormal situation occurs kinds of sensors include a level.