1. An image processing device comprising: a decoding unit configured to decode a bit stream encoded in a unit having a hierarchical structure, and generate an image; a filtering unit configured to perform a filtering operation on the image generated by the decoding unit; and a filter adjustment unit configured to adjust a parameter of the filtering unit so as to perform a strong filtering operation on a boundary between a current region to be processed in the image generated by the decoding unit and a neighboring region adjacent to the current region, when a difference value between a quantization parameter of the current region and a quantization parameter of the neighboring region is larger than a threshold value.