It is an object of the present invention to provide an image coding technique for suppressing degradation in image quality, in which the time and space where intra macroblocks appear are dispersed. A numerical value (Ftk) is generated from the lower-order six bits of the frame number (Ft) of a coding object frame. A numerical value (Fs) is generated by shifting the numerical value (Ftk) leftward by two bits. An exclusive OR of the numerical value (Ftk) and the numerical value (Fs) is calculated, to thereby generate a numerical value (A). A numerical value (Ytk) is generated from the lower-order six bits of the Y coordinate (Yt) of a coding object macroblock. The upper-order bits of the numerical value (Ytk) and the lower-order bits thereof are inverted, to thereby generate a numerical value (Yr). Further, an exclusive OR of the numerical value (Yr) and the numerical value (A) is calculated, to thereby generate a numerical value (B). A numerical value (Xtk) is generated from the lower-order six bits of the X coordinate (Xt) of the coding object macroblock. When the numerical value (Xtk) and the numerical value (B) are identical to each other, the coding object macroblock is intra-coded.