An infrared ray detector comprises a prism element, a condenser lens, and an infrared ray receiving unit. The prism element is configured to convert the infrared ray irradiated from a detection area of a viewing field to the infrared ray proceeding toward the condenser lens. The condenser lens is configured to concentrate the infrared ray into the infrared ray receiving unit. The infrared ray receiving unit includes a plurality of the infrared ray detection elements. The infrared ray detection elements are arranged in an alternate fashion so as to output electrical signals of positive polarity and negative polarity. Consequently, the infrared ray detector is configured to detect the infrared ray irradiated from a plurality of the detection area, and is configured to detect the infrared ray on the basis of movement of the human in the detection area.