1. A working vehicle periphery monitoring system that monitors a periphery of a working vehicle including a vehicle body frame which supports front and rear wheels and a vessel which protrudes toward a rear side of the vehicle body frame than the rear wheels to load freight thereon, the working vehicle periphery monitoring system comprising: a plurality of image capturing devices each of which is attached to the working vehicle and captures the periphery of the working vehicle to output a piece of image information; a bird's eye image synthesizing unit that acquires a plurality pieces of the image information and synthesizes the acquired image information to generate bird's eye image information for displaying a bird's eye image of the working vehicle and the periphery of the working vehicle on a display device; and a display control unit that displays, on the display device, at least one of a first image obtained by superimposing a first vessel image including at least the vessel other than a part of the vessel at the rear side of the working vehicle on an area in which the working vehicle is present in the bird's eye image and a second image obtained by superimposing a second vessel image including at least all of the vessel on the area in which the working vehicle is present in the bird's eye image.