A method for converting frames that can display smooth moving images is provided. A memory (16) to which image data of frames in the NTSC format is written is provided. A memory controller (14) for retrieving image data of an odd field and an even field from the memory (16) every odd field period and even field period in the frame period of the PAL format is provided. An interpolating circuit (76) is provided. The interpolating circuit (76) mixes the image data of the odd field retrieved by the memory controller (14) and image data of a next odd field at a predetermined ratio to output as image data of an odd field in the frame period of the PAL format, and mixes the retrieved image data of the even field and image data of a next even field at a predetermined ratio to output as image data of an even field in the frame period of the PAL format. A coefficient-generating circuit (73) for changing the mixing ratios in the interpolating circuit (76) every field period of the PAL format is provided.