PURPOSE: To prevent erroneous judgement such that a body is present though it is absent by allowing infrared rays emitted from an arbitrary body to be incident to a hologram element and spectrally diffracting the same into a plurality of wavelengths by the hologram element.

CONSTITUTION: A part of infrared rays emitted from a body is incident to a hologram element 1. The infrared rays passed through pinholes 2a, 2b, 2c are respectively detected by detection elements 3a, 3b, 3c. Then, the value Va of a voltage signal is compared with the value Vb of a voltage signal. In the case of Va < Vb and Vb > Vc, the center wavelength of infrared rays can be judged to be within a range of 9 - 11 μm. In this case, since possibility such that the center wavelength is in the vicinity of 10 μm is highest, it is assumed that a human being is present and the presence thereof is displayed on the display part 6 connected to a microcomputer 5.