1. A moving image encoding device, comprising: a processor; and a memory which stores a plurality of instructions, which when executed by the processor, cause the processor to execute, obtaining a region motion vector that represents a motion of a region including a block in a first slice, in a case where an encoding object block is included in a block line in a second slice, the block line contacting with a boundary between the first slice and the second slice in an encoding object picture, the encoding object picture being divided into a plurality of slices each of which includes a plurality of blocks; obtaining a motion vector of the encoding object block by using the region motion vector as a first prediction vector of the encoding object block; obtaining a second prediction vector from a motion vector of one or more encoded blocks that is or are positioned around the encoding object block; and obtaining an encoding result by encoding a difference vector between the motion vector of the encoding object block and the second prediction vector and encoding a difference between a pixel value of the encoding object block and a pixel value of a block in a reference picture that is represented by the motion vector of the encoding object block.