A smoothing process is performed on image data to obtain improved image quality. Output image data is switched among image data subjected to smoothing, image data subjected to second bit conversion, and image data subjected to pseudo halftone representation process. In this manner, jaggies at edges can be smoothed even in multilevel image data subjected to image forming process such as screen processing. Smoothing for image data with degraded image quality due to, for example, lossy compression such as JPEG compression can also be achieved. Furthermore, degradation in image quality that occurs when a conventional smoothing process is applied to image data subjected to particular image processing can be avoided.