An image processing apparatus comprises: a decision unit which decides, based on a first resolution and a second resolution, a sampling pixel to be used for resolution conversion in the first image data; a determination unit which determines whether the first image data includes a white line of not larger than a predetermined width; a replacing unit which, in a case where a pixel of interest in the first image data is a pixel included in the white line and is not the sampling pixel, replaces a value of the pixel of interest with a value of a pixel adjacent to the pixel of interest; and a conversion unit which converts the first image data including the replaced value of the pixel into the second image data having the second resolution.