1. An obstacle detection apparatus that detects an obstacle of which edges are difficult to be detected, the apparatus comprising: a processor programmed to: estimate, as an existence region, in which an obstacle exists, in an image captured by an image capturing unit, two regions with shapes similar to each other that indicate brightness greater than or equal to a predetermined threshold in the captured image, the image capturing unit being provided on a vehicle to capture images of surroundings of the vehicle; and in response to the two regions with shapes similar to each other that indicate brightness greater than or equal to the predetermined threshold in the captured image being estimated as the existence region, determine that the obstacle is present in the existence region in a case where the two regions are extended and brightness of both of the two regions are increased, depending on movement of the vehicle.