1. An image alignment apparatus comprising a hardware processor to implement: a first conversion function estimation unit configured to estimate a first conversion function based on feature point information, the feature point information being extracted from a first image, which is captured by using a first image sensor, and a second image, which is captured by using a second image sensor that is different from the first image sensor; a second conversion function estimation unit configured to estimate a second conversion function based on a similarity between a first motion vector in the first image and a second motion vector in the second image; and an alignment unit configured to align the first image with the second image by using the first or second conversion function.