It is an object to generate a desired composite image in which a motion area of a subject is correctly composited.A difference image between a base target and an aligned swap target is generated (S1021), and an extracted contour in the difference image is determined according to the active contour model (S1022). The inner area of the contour and the outer area of the contour are painted with different colors to be color-coded so as to generate a mask image for alpha blending (S1023). Using the mask image thus generated, the swap target that is aligned with respect to the base target is composited with the base target of the base image by alpha blending (S1024).