To provide an image compression data decoder capable of easily writing image data subjected to decoding and resolution conversion to a display memory and decreasing disturbance in a decoded image caused on the occurrence of a decoding error of a variable length code.

In order to reduce the memory capacity, image data are stored in a display memory 16 after reducing the image data by using resolution conversion resulting in reducing the memory capacity. The decoder is provided with a macro block number output section 14 and a display memory write control section 15 to write the image data on the way of a line buffer in the unit of macro blocks and is configured not to overwrite ineffective data to data of the display memory equivalent to a block earlier than an erroneous packet in the display memory thereby making disturbance of a decoded image unremarkable with the memory capacity as small as possible on the occurrence of a decoding error of the variable length code.