1. A image processing apparatus, comprising: an image sensor unit that images, in a given frame period, at least a long-accumulation image and a short-accumulation image using different exposure times; and an image processing unit that: determines a first exposure evaluation value from the long-accumulation image using a first exposure evaluation method, determines a second exposure evaluation value from the long-accumulation image using a second exposure evaluation method, determines a third exposure evaluation value from the short-accumulation image; determines a target short-accumulation exposure time based on the third exposure evaluation value, selects one of the first and second exposure evaluation values based on the target short-accumulation exposure time, determines a target long-accumulation exposure time based on the selected one of the first and second exposure evaluation values, and synthesizes the long-accumulation image and the short-accumulation image into a composite image.