PURPOSE: To improve the alarming accuracy of a human body detecting sensor by measuring the turning-on time and number of detecting area passing times of persons on the basis of the output of the sensor and changing the discriminating criterion of the sensor in corresponding to time zones.

CONSTITUTION: The detecting area of this burglar sensor is divided into numerous small areas and, when a person arrives and passes through one of the small areas, a pyroelectric sensor 1 gives a signal to a off-delay timer 3 and a time measuring section 5 measures the turned on time of the sensor 1. On the other hand, a counter 4 counts the number of pulses from the sensor 1. A clocking controller 8 outputs the signal of either a business time zone or working time excluding the business time and unmanned time zone in night time and on holidays in accordance with the current time. Input correcting sections 6 and 7 correct input values by multiplying input values by factors corresponding to the frequency of occurrence of alarms in the time zones. A fuzzy reasoning section 9 performs fuzzy reasoning following a prescribed fuzzy rule and outputs a signal indicating the degree of doubtfulness of an arriving person.