PURPOSE: To distinguish only the target having a heat source, by using two kinds of wavelength bands, and cancelling the effect of solar reflected light.

CONSTITUTION: The wavelengths of incident light beams R1 and R2 are divided by a dichroic mirror M in an optical system 5. The light beam R1 having the wavelength of about 4μm is transmitted through the mirror M and inputted to a light detecting element 6. Meanwhile, the light beam having the wavelength of about 10μm is reflected and inputted to a light detecting element 7. The outputs of the light detecting elements 6 and 7 are amplified by a pre-amlifier 8, and the corresponding picture elements are subjected to subtraction in a differential amplifiers 9. At this time, each signal is adjusted to such a level that a body based on the solar reflected light disappears on a display monitor.