1. An imaging device comprising: an infrared imaging means for imaging an area under surveillance by infrared radiation measurement; a moving-subject detecting means for detecting a moving subject in the surveillance area on the basis of an infrared image signal acquired by the infrared imaging means; an image signal processing means for coloring a moving part of the infrared image signal, detected by the moving-subject detecting means, with a predetermined color; and an image displaying means for displaying an image resulted from the coloring of the moving part of the infrared image signal by the image signal processing means.