1. A resolution determination device, comprising: an edge strength calculator configured to obtain an edge strength of a pixel included in an input image based on luminance of the pixel and luminance of a pixel adjacent to the pixel, for each of a plurality of pixels included in the input image; and a resolution determiner configured to determine whether or not the input image is an image upconverted from an image with a predetermined resolution or less, based on distribution of the edge strengths, and output a determination result, wherein the resolution determiner includes: a first counter configured to generate a count representing the number of pixels whose edge strengths are within a first range; a second counter configured to generate a count representing the number of pixels whose edge strengths are within a second range; a selector configured to select and output the count of the first counter or the second counter based on an input parameter; and a correction factor generator configured to generate a correction factor based on the count selected by the selector and output the correction factor as the determination result.