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, each of the cameras being configured and arranged to obtain a camera image capturing a portion of the surrounding of the work vehicle, the camera images together forming 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 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 which of the camera images captured by the cameras include at least one obstacle detected by the obstacle detecting sensors, and to determine whether there is one or a plurality of the camera images including at least one obstacle; and a camera image displaying unit configured to select the one specified camera image when the camera image specifying unit specifies only one of the camera images, and to select a most relevant one of the plurality of camera images based on a priority order when a plurality of the camera images are specified by the camera image specifying unit, the camera image displaying unit displaying the selected camera image separately from and adjacent to the bird's-eye image on the display device, the most relevant camera image being a camera image having a highest priority ranking among the plurality of specified camera images based on the priority order, the priority order being set in accordance with a travelling state of the work vehicle.