PURPOSE: To maintain an S/N even when videos require definition by increasing the number of output ports of a detector in proportion to the definition.

CONSTITUTION: The output signal of a detector 1 is frame-integrated 6 after the output signal is digitized 4 and the sensitivity fluctuation among multiple detecting elements is corrected 5. When the number of detecting elements of the detector 1 is increased by (m) times to improve the definition of infrared videos, the number of output ports of the detector 1 is increased by (m) times. When the definition of infrared videos is improved by (m) times without changing the number of output ports of the detector 1, the number of detecting elements corresponding to the same width of an object becomes (m) times larger and the output rate of the detector 1 in which the outputs of the detecting elements are connected in series drops to 1/m. Therefore, the adding number of frame integration is reduced to 1/m and the S/N drops. When the number of output ports is increased by (m) times, on the contrary, the number of detecting elements per one output port does not change and the output rate of each output port becomes the same as that obtained when the definition is not improved. Therefore, the number of frame integration does not change and the definition is improved and, at the same time, a high S/N can be secured.