1. An image processing apparatus, comprising: a registration unit for executing a registration process among a plurality of images; an image synthesis unit for superposing the plurality of images subjected to registration in the registration unit to synthesize the plurality of images and generate a synthetic image; a frequency characteristic measurement unit for measuring a frequency characteristic of the synthetic image; and a registration error detection unit for detecting an error of the registration process among the plurality of images based on the measured frequency characteristic of the synthetic image.