An image processing device includes: an image acquisition unit configured to acquire first and second image groups in different first and second directions; a flatness calculation unit configured to calculate first flatness representing a gradient of a shading component in the first direction and calculate second flatness representing a gradient of a shading component in the second direction; a flat area detection unit configured to detect an area, as a flat area, including a position having a minimum gradient of a shading component in an image based on the first and second flatness; and a correction gain calculation unit configured to calculate a correction gain for correcting shading in an image with reference to a correction gain in the flat area by using a luminance ratio in the common area.