1. An image processing apparatus, comprising: a processor configured to execute a process including: detecting regions in which a pixel value is changed between frames included in moving image data; selecting a region whose a filling ratio of the region to a circumscribed rectangle of the region is greater than or equal to a threshold among the regions; and specifying a frame including a detection target based on a filling ratio of the region selected at the selecting, and a size of a plurality of corner regions surrounded by the circumscribed rectangle and the region.