1. A periphery-monitoring device for a working machine, comprising: a camera to be mounted to the working machine to acquire an image of an object in a periphery of the working machine as an input image in an input image plane; a distance image sensor to be attached to the working machine to acquire distance information of the object as an input distance image in an input distance image plane, the distance image sensor including a first distance image sensor and a second distance image sensor that are configured to acquire the distance information of the object as a first input distance image and a second input distance image, respectively, wherein there is an overlap of an imaging range between the first and second distance image sensors; and a computer programmed to execute a process, the process including converting the input image into an output image in an output image plane based on correspondence between coordinates of the input image plane and coordinates of the output image plane; creating an output distance image based on the first input distance image and the second input distance image by converting the first input distance image and the second input distance image into the output distance image in an output distance image plane based on correspondence between coordinates of the input distance image plane and coordinates of the output distance image plane, wherein a pixel of the output distance image in an area corresponding to the overlap of the imaging range is associated with a smaller one of a pixel value of a first pixel of the first input distance image and a pixel value of a second pixel of the second input distance image, the first pixel and the second pixel corresponding to said pixel of the output distance image; and displaying the output image and the output distance image in an overlapping manner.