1. An image processor comprising: an image input unit configured to input a multiple image including superimposed images in which images of a subject are superimposed on each other; a frequency transformation unit configured to perform frequency transformation processing with respect to the input multiple image; and an inter-superimposed-image displacement calculation unit configured to calculate a displacement amount between images forming the superimposed images included in the input multiple image by using a frequency-transformed image subjected to the frequency transformation processing.