1. An object detection apparatus comprising: means for acquiring image data from an in-vehicle imaging device, the in-vehicle imaging device being configured to image an environment of a vehicle and output the image data that is indicative of the imaged environment; and means for extracting an object region from the image data acquired by the acquiring means, the object region being an image region where a preset object exists, wherein the object region extracting means comprises: means for identifying the object region, the identifying means including a plurality of identifiers for identifying candidate regions for the object region, the identifiers being serially linked so as to sequentially refine the candidate regions; means for measuring processing time of the identifying means for processing the image data acquired by the acquiring means to identify the object region; and means for controlling, based on the measured processing time at a preset time point, a degree of refining the candidate regions by the identifying means after the preset time point by adjusting at least one of the linkage between the identifiers and processes to be executed by the identifiers after the preset time point.