To surely make a file size of encoded image data smaller than a desired file size when encoding image data.

An encoding parameter is set to an initial value, image data are encoded, and it is discriminated whether or not, the size of the encoded image data becomes smaller than a specified file size. If a result of said discrimination is denial, it is judged whether the parameter is a critical parameter. If a result of said judgement is denial, the parameter is changed, and image data are encoded by the changed parameter. If a result of judging whether the parameter is the critical parameter is positive, an image size of the image represented by the image data is reduced, so that a file size of encoded image data becomes smaller than the specified file size.