1. An image reproducing method of decoding a moving picture stream having been subjected to inter-picture prediction encoding with accessing a reference image memory, the method comprising: performing first decoding for decoding the moving picture stream on a picture-by-picture basis to estimate traffic to the reference image memory on the picture-by-picture basis; and performing second decoding for decoding the moving picture stream on the picture-by-picture basis to generate a reproduction image, wherein: when a picture to be decoded is to be stored as a reference image in the reference image memory after the picture is decoded, in the first decoding to estimate traffic, it is determined whether compression of a reference image is needed in advance, and in the second decoding, if it is determined that compression of the reference image is needed, a compression mode in storing the picture to be decoded in the reference image memory is determined in advance based on the traffic for a picture referring to the picture to be decoded, which has been estimated in the first decoding, and a compressed image of the picture to be decoded is stored in the reference image memory, and if it is determined that compression of the reference image is not needed, a non-compressed image of the picture to be decoded is stored in the reference image memory.