PURPOSE: To make it possible to measure temp. with good accuracy even when the emissivity of an object to be measured is unknown, by respectively measuring the levels of a large number of wavelength components of infrared rays emitted from the object to be measured.

CONSTITUTION: A large number of optical fiber cores 2a... capable of transmitting infrared rays from an object A to be measured incident through a condensing convex lens 1 are bundled so as to show a circuit cross-sectional area as a whole to constitute an optical fiber bundle 2. Variable filters 3, 3 different in a transmitting wavelength corresponding to a position are provided to the incident end 2A and emitting end 2B of infrared rays in such a state the relative positions to the incident and emitting ends 2A, 2B are unchangeable. A plurality of sensors 4a... capable of detecting the levels of infrared rays emitted from the predetermined number of cores 2a are provided to the rear end of the emitting end 2B in parallel to constitute a sensor array 4 which is, in turn, arranged in such a state that the relative position to the emitting end 2B is unchangeable. On the basis of a large number of level measuring results, the whole configuration of an infrared spectrum is identified to accurately detect temp. even when emissivity is unknown.