1. A periphery monitoring device for a work machine, comprising: a plurality of cameras that each captures an image of surroundings of the work machine; an image generator that is connected to the plurality of cameras, converts the image captured by each of the plurality of cameras into an upper view-point image, and generates an overhead view image of the surroundings of the work machine based upon upper view-point images; and a display monitor that is connected to the image generator and displays the overhead view image output from the image generator in an overhead view image display region, wherein when generating an overhead view image of an overlap region of a first upper view-point image relating to the image captured by a first camera included in the plurality of cameras and a second upper view-point image relating to the image captured by a second camera included in the plurality of cameras, the image generator, based upon a height of a virtual monitoring target, sets a first region in which only the first upper view-point image is displayed in the overlap region and a second region in which only the second upper view-point image is displayed in the overlap region, and also sets a third region in which a composite display image based upon the first and second upper view-point images is displayed in the overlap region.