There is provided with a decoder which decodes coded image data obtained by applying entropy coding to first data obtained by applying a discrete cosine transform and quantization processing to image data, including: a first decoding unit configured to apply entropy-decoding to the coded image data to obtain the first data; a dividing unit configured to divide the first data into a plurality of second data each of which has a size equal to or smaller than a first threshold; and a second decoding unit configured to apply inverse quantization processing and inverse discrete cosine transform to each second data sequentially to obtain a plurality of third data, and a generating unit configured to generate the image data by synthesizing the plurality of third data.