An imaging device includes an imaging unit mounted on a vehicle and obtaining a vertically-polarized image and a horizontally-polarized image of a road surface on which the vehicle is running; a polarization ratio image generating unit generating a polarization ratio image and calculating polarization ratio information indicating polarization ratios of pixels of the polarization ratio image based on the vertically-polarized image and the horizontally-polarized image; and a roadside structure detection unit detecting a planar line formed on and partitioning the road surface and/or a roadside structure located adjacent to and at an angle with the road surface based on the polarization ratio information of the polarization ratio image.