1. An image encoding apparatus comprising: a processor configured to prepare a prediction for each of a plurality of intra-prediction modes of an encoding target block by using at least one of pixel signals of pixels of the encoding target block lined horizontally in an upper end of the encoding target block and pixel signals of pixels of the encoding target block lined vertically in a left end of the encoding target block; and execute an intra-prediction operation of the encoding target block by using an image of the encoding target block in an encoding target image and the prepared prediction, wherein when the preparing step uses pixel signals of the pixels of the encoding target block lined horizontally in the upper end of the encoding target block, the preparing step prepares the prediction by further using pixel signals of pixels lined vertically in a right end of a block adjacent to a left side of the encoding target block in the encoding target image, and when the preparing step uses pixel signals of the pixels of the encoding target block lined vertically in the left end of the encoding target block, the preparing step prepares the prediction by further using pixel signals of pixels lined horizontally in a lower end of a block adjacent to an upper side of the encoding target block in the encoding target image.