Image encoding/decoding apparatus and method are disclosed. The image encoding apparatus includes: a target block splitter for splitting a target block of an input image to be encoded into a plurality of subblocks according to positions of pixels; a key block encoding unit for selecting a random key block, the size of which is larger than a pixel size of at least one of the plurality of subblocks split by the target block splitter, and performing an intra-prediction or inter-prediction encoding process on the selected key block; and a non-key block encoding unit for designating the rest of the subblocks, except for the selected key block, as a non-key block, and performing a spatial prediction process on the designated non-key block by using a key block reconstructed by the key block encoding unit and values of peripheral pixels of a current block.