1. A field watch apparatus comprising: an image capture unit capturing a field image around a vehicle; an obstacle detector detecting an obstacle around the vehicle; a display unit displaying the field image captured by the image capture unit, wherein the field image has an obstacle mark superposed thereon when the obstacle is detected by the obstacle detector; a data acquisition unit continually acquiring obstacle data from the obstacle detector after an initial detection of the obstacle; a storage unit accumulating the obstacle data continually acquired by the data acquisition unit; and a change unit changing the obstacle mark in association with the obstacle data, wherein the change unit displays multiple obstacle marks on the field image based on multiple pieces of the obstacle data in the storage unit, and an obstacle mark representing an older piece of obstacle data is changed from an obstacle mark representing a less older piece of obstacle data for the ease of distinction in the field image.