1. An object detection apparatus comprising: an obtaining portion that obtains a captured image of a scene around a host vehicle, information indicating a travel state of the host vehicle, and information indicating a surrounding situation around the host vehicle; a risk area estimation portion that estimates risk areas in each of which an object is likely to exist, and area risk degrees indicating risk degrees of the risk areas, based on the information obtained by the obtaining portion; a posture risk degree estimation portion that estimates posture risk degrees indicating risk degrees of respective postures, based on a relation between a positional relation between each of the risk areas and the host vehicle, and the postures of the object assumed to exist in the risk area; an extraction portion that estimates an overall risk degree of each of the postures of the object assumed to exist in each of the risk areas, based on the corresponding area risk degree and the corresponding posture risk degree, selects the risk areas according to descending order of the overall risk degrees, and extracts windows from areas in the captured image, the areas respectively corresponding to the selected risk areas; and an identification portion that identifies whether each of the window images extracted by the extraction portion is an image showing the object, based on an identification model and the each of the window images, the identification model being selected from identification models generated in advance for the respective postures of the object to identify the object, in accordance with the posture of the object assumed to exist in the corresponding selected risk area, wherein each time when one of the risk areas is selected by the extraction portion, the extraction portion extracts the window image from the corresponding one of the areas in the captured image and the identification portion identifies whether the extracted window image is the image showing the object.