Rendering artifacts

Hi. How can I fix these rendering artifacts(gradient lines)? Thanks

p.s blender file is included. (87 KB)

Render > Postprocessing > Dither:

disregard… same as above, but late

Thanks. It worked. But it have a litte noise. How can I remove that? Thanks again.

Well, dithering adds noise to avoid banding.
You can either have noise or banding, but not avoid both (when using 8 bit image formats).
The alternative would be to save as 16 bit PNG or EXR, but then only some programs will be able to read the image.

Thank you for the explanation.