1. A moving image region detection device, comprising: a memory containing program instructions; and a processor coupled to the memory, wherein the processor is configured to execute the program instructions to: compare, with respect to a frame of an input video, pixel values between the frame and a previous frame of the frame; detect, from among a plurality of blocks formed by dividing the frame into blocks, a block including a pixel in which a value has been changed as an updated block; calculate, with respect to the updated block of the frame, an index value representing a degree of continuity of changes in pixel values in the updated block; compare the calculated index value with a threshold; determine whether or not the updated block is an updated block of a natural image; extract, with respect to the frame, a block determined to be an updated block of a natural image a given number of times or more in a plurality of most recent frames including the frame, as a moving image block of a natural image; link, with respect to the frame, the moving image blocks of a natural image, which are spatially adjacent to each other; acquire a circumscribed rectangle of the linked moving image blocks of the natural image as a moving image region seed of the natural image; search for and determine, with respect to four edges on top, bottom, left, and right of the moving image region seed of the natural image, a boundary of the moving image region of the natural image in pixel units; attempt to expand, in block units, each of the four edges on top, bottom, left, and right of the moving image region of the natural image given in block units; calculate, with respect to a block which may be newly added to the moving image region of the natural image by expansion, the average number of times that the block has been determined to be an updated block of a natural image within a given number of most recent frames; and add the block to the moving image region of the natural image if a calculated value exceeds a given value.