A circuit for converting the format of image data, the motion of the image data being smooth, includes a memory (16) to which moving image data in the NTSC format is written; a memory controller (14) retrieving respective signals, from the memory (16), required for producing image data of an odd field and an even field in the PAL format; line-interpolating circuits (73) and (74) converting retrieved image data into first image data and second image, respectively, both image data having the line frequency of the PAL format; a frame-interpolating circuit (75) outputting the image data of the odd field in the PAL format by mixing image data of the odd field of the first image data and image data of the odd field of the second image data at a predetermined mixing ratio, and outputting the image data of the even field in the PAL format by mixing image data of the even field of the first image data and image data of the even field of the second image data at a predetermined mixing ratio; and a coefficient-generating circuit (77) changing the mixing ratios every field period in the PAL format.