There is provided an intra prediction system having high prediction performance and satisfactory encoding efficiency in a pattern image in which pixel value is periodically changed and a pattern image in which pixel value has a constant change tendency like gradation effect. A frequency component extraction unit allows a pixel value of an encoded MB to be made one-dimensional for one-dimensional orthogonal conversion. A frequency component-considering prediction value calculation unit receives, as inputs, a pixel value of an encoded adjacent MB and an alternating current frequency component output to create an orthogonal conversion coefficient. The orthogonal conversion coefficient then is subjected inverted orthogonal conversion (IDCT) to generate a prediction value p(i). A prediction direction determination unit selects and outputs a prediction value p′ (i) in a prediction direction having the highest encoding efficiency among a conventional prediction value calculation unit and a prediction value calculation unit according to the present invention.