1. A moving image region detection device, comprising: an updated block detection unit that, with respect to a frame of an input video, compares pixel values between the frame and a previous frame of the frame, and from among a plurality of blocks formed by dividing the frame into blocks, detects a block including a pixel in which a value has been changed as an updated block; a natural image updated block determination unit that, with respect to the updated block of the frame, calculates an index value representing a degree of continuity of changes in pixel values in the updated block, compares the calculated index value with a threshold, and determines whether or not the updated block is an updated block of a natural image; and a moving image block extraction unit that, with respect to the frame, extracts 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.