1. An image processing device for identifying a characteristic of an eye from a face image of a person, the image processing device comprising: a first differentiation unit configured to differentiate an eye region where the eye of the face image exists in at least a vertical direction of the eye to obtain a first luminance gradient; a first edge extraction unit configured to extract a first edge point in the eye region according to the first luminance gradient; and a curve identification unit configured to identify a curve, which is expressed by a control point and both end points and fits to the first edge point, as a curve expressing an upper-eyelid or lower-eyelid outline, the end points being an inner corner point of eye and a tail point of eye, wherein the curve expressed by both the end points and the control point is a B-spline curve or a Bezier curve, wherein the curve identification unit is configured to vote for the control point that is a voting target with respect to the first edge point using the Hough transform, and identify the B-spline curve or the Bezier curve expressed by both the end points and at least one of the control point, which fits to the first edge point, as the curve expressing the upper-eyelid or lower-eyelid outline based on the voting result.