In an exposure determining device, an image acquiring unit acquires a first image. The first image is picked up by an image pickup unit at a first exposure. A light-intensity detecting unit extracts areas in the first image and detects a light intensity in area. A mismatch area determining unit determines, based on the detected light intensity in each area, whether each area is a mismatch area in which an edge is estimated to be undetected due to excess or shortage of the first exposure for a corresponding one of the areas. When it is determined that one of the areas is the mismatch area, a second exposure determining unit determines a second exposure of the image pickup unit. The second exposure is matched with the light intensity in the mismatch area.