An image processing apparatus that is capable of using frame information associated with image data effectively for an image that is formed by trimming the image shown by the image data. The image processing apparatus processes image data with which frame information about a partial area in an image is associated. A trimming unit cuts out a selection area as a trimming image from an image represented by the image data. A determination unit determines whether frame information updated according to the area cut out by the trimming unit is added to the trimming image or not based on a determination of whether a specific frame effective area in the area shown by the frame information is included in the trimming image or not.