PURPOSE: To easily remove an error factor and to increase the reliability of the title detector by a method wherein an electric signal at a level corresponding to the energy amount of infrared rays in a detection area is input, the change amount per unit time of an input is compared with a set value and whether a human body exists or not is judged.

CONSTITUTION: An A/D conversion part 151 in a judgment device 15 converts an input signal into a digital value. A judgment part 152 operates the change amount of the digital value at every unit time, and compares it with set values +DH1, -DH1. When the change amount exceeds the set value +DH1, it is judged that a human being has entered a detection area 10, and a human-body detection signal is generated. When the change amount exceeds the set value -DH1, it is judged that the human being has left the area 10, and the generation of the human-body detection signal is stopped. Since the change amount per unit time is compared with the set values in this manner, it is possible to prevent an erroneous detection due to a noise caused by a pyroelectric infrared detection element 12 itself and to prevent a malfunction caused by a signal processing system and a malfunction caused by a change in surroundings, and the reliability of the title apparatus is enhanced.