1. An image compressing device comprising: a prefetched-image memory that stores a prefetched image of a prefetched area prefetched from a reference image memory that stores a reference image; and a processor that performs a motion search using a reduced image obtained by reducing an encoding target image and generates a search result for an encoding target block included in the encoding target image, determines, on the basis of the search result, that an area situated outside the prefetched area in the reference image is to be searched when the processor has predicted from the search result that a matching of the encoding target block and the prefetched image will be unsuccessful, performs a matching of the encoding target block and an image of a matching target area including the outside area in the reference image when the processor has determined that the outside area is to be searched and generates an inter-prediction result, and encodes the encoding target block on the basis of the inter-prediction result.