1. A perimeter monitoring device for a work vehicle configured to monitor a surrounding of the work vehicle and to display a monitored result on a display device, the perimeter monitoring device comprising: a plurality of cameras attached to the work vehicle, the cameras being configured and arranged to a plurality of sets of image data regarding the surrounding of the work vehicle; a bird's-eye image display unit configured and arranged to display a bird's-eye image of the surrounding of the work vehicle on the display device based on the sets of image data obtained by the cameras; a plurality of obstacle detecting sensors attached to the work vehicle, the obstacle detecting sensors being configured and arranged to respectively detect obstacles in the surrounding of the work vehicle; a camera image specifying unit configured to specify a single or plurality of camera images in which a single or plurality of obstacles are captured when the single or plurality of obstacles are detected by the obstacle detecting sensors; and a camera image displaying unit configured to display a relevant camera image in alignment with the bird's-eye image on the display device when a plurality of camera images are specified by the camera image specifying unit, the relevant camera image being ranked in a high priority ranking based on a priority order set in accordance with travelling states.