1. A periphery-monitoring device for working machines that creates an output distance image based on an input distance image, the periphery-monitoring device for working machines comprising: a distance image sensor attached to a working machine to acquire the input distance image; and a computer programmed to execute a process including: obtaining coordinates on an input distance image plane on which the input distance image is positioned; obtaining coordinates on an output distance image plane on which the output distance image is positioned; and associating the input distance image and the output distance image with each other.