A coding parameter extracting unit extracts and supplies coding parameters of a code stream to an image characteristic amount output unit. A code stream characteristic amount extracting unit outputs a first vector calculated based on the number of zero bit planes for each code block extracted by analyzing the code stream. The image characteristic amount output unit uses the coding parameters other than a quantization step size of the code stream to output a second vector calculated based on the number of zero bit planes for each code block of a specific image. A comparing unit compares the first and second vectors for each code block and makes a matching decision between the code stream and the specific image.