To prevent deterioration in image quality of a decoded image obtained by encoding and decoding an image to which editing such as dithering is applied.

A prediction unit 1 determines a prediction error between a source image and a predictive image, a conversion unit 2 converts the prediction error into a frequency component, and a quantization unit 43 quantizes the frequency component. On the other hand, a dithering information acquisition unit 41 acquires dithering information representing whether dithering is applied to the source image and on the basis of the dither information, a dithering quantization step control unit 42 controls a quantization step in the case where the quantization unit 43 quantizes the frequency component. The present invention may be applicable to an encoder which encodes an image using an encoding method compliant to H. 264/AVC or the like.