1. An imaging device comprising: a filter that includes a plurality of filter areas having different spectral transmission characteristics; an imaging element to receive light transmitted through the filter and output image information; a lens array that includes a plurality of lenses arranged approximately parallel to a light receiving surface of the imaging element and that is arranged between the filter and the imaging element; a spectral image generator to generate a plurality of spectral images respectively corresponding to the plurality of filter areas on the basis of the image information output by the imaging element; and an exposure adjuster to adjust an exposure time of the imaging element on the basis of luminance values of the spectral images.