PURPOSE: To enable early and reliable detection of the presence or absence of a human body by detecting and storing the maximum value of each output of an infrared ray detecting element from the time of output to the present and by comparing the maximum value of each output with the time of output mutually.

CONSTITUTION: A decision unit 5 detects times tA to tD when outputs of infrared ray detecting elements A to D processed in a signal processing unit 4 exceed a threshold value, and a maximum value detecting unit 7 detects and stores maximum values XA to XD of the outputs from the times tA to tD to the present time. The decision unit 5 checks up whether or not a first determining condition TMIN < t< TMAX (where TMIN and TMAX denote the lower and upper limits of a time difference t from tA to tD) on a human body is satisfied, and when it is not satisfied, the unit ends a processing without issuing an alarm. When the condition is satisfied, the unit reads the maximum values XA to XD from the detecting unit 7 and checks up whether or not a second determining condition XMAX/XMAX > S (where XMIN and XMAX denote the minimum and maximum values of XA to XD, and S the threshold value) is satisfied. For instance, the alarm is issued at a time TE since the difference between XA and XD is small enough to satisfy the condition thereat, while the difference is too large to satisfy the condition at a time T. When the condition is not satisfied, the same processing is repeated.