1. An imaging device comprising: a far-infrared camera configured to capture an image of a scene using a far-infrared ray in a first wavelength band and generate a far-infrared camera image, the first wavelength band being included in a first range from 4 to 1000 μm; a near-infrared camera configured to capture an image of the scene using a near-infrared ray in a second wavelength band shorter than the first wavelength band and generate a near-infrared camera image, the second wavelength band being included in a second range from 0.7 to 2.5 μm; a visible light camera configured to capture an image of the scene by visible light and generate a visible light camera image; a low-level region detection unit configured to detect a low-level region with a level or less based on a threshold value from the far-infrared camera image; a transparent region detection unit configured to detect a low-level region as a transparent region based on both the near-infrared camera image and the visible light camera image taken as a portion of at least one of the near-infrared camera image and the visible light camera image, the low-level region having the level or less resulting from being a transparent region that is a region covered with a visually transparent material, within the low-level region detected by the low-level region detection unit; an image segmentation unit configured to segment a portion of the transparent region from the near-infrared camera image; and a superimposition unit configured to superimpose the portion of the transparent region onto a position of the far-infrared camera image, the position corresponding to a position of the transparent region, and generate a synthetic image, wherein the transparent region detection unit converts frequency of a signal of the visible light camera image, and determines that the low-level region is a transparent region, when a level of the near-infrared camera image is a predetermined level or greater, and when the frequency-converted signal of the visible light camera image includes a high frequency component with a reference value or greater.