A captured image displaying device generates a video signal for causing a thumbnail image of a video taken by an imaging device to be displayed on a map. A synthesis unit generates a video signal for synthesizing and displaying a thumbnail image of a video of an imaging device in an image display area provided in association with an imaging position of the imaging device on a map. When there are imaging positions of a plurality of imaging devices in the image display area, the synthesis unit generates a video signal that causes thumbnail images of videos of the plurality of imaging devices to be switched and displayed in the image display area in a predetermined sequence, or generates a video signal that splits the image display area and causes the thumbnail images of the videos of the plurality of imaging devices to be displayed in split areas.