1. An image processing apparatus comprising: an imaging device configured to obtain a radiographic image according to radiation; and at least one processor operatively coupled to a memory device and programmed to provide: a defect information storage unit configured to store a first defective pixel that outputs an abnormal value regardless of time to a defect map; a defect information obtaining unit configured to obtain, based on characteristic information indicating a relationship between an output fluctuation amount of a pixel of the imaging device and an elapsed time after the imaging device is ready for imaging, a second defective pixel in which the output fluctuation amount under an unexposed state exceeds a predetermined threshold, wherein the defect information storage unit stores the second defective pixel to the defect map; and a defect correction unit configured to correct defects of the radiographic image based on the defect map that includes the first defective pixel and the second defective pixel.