1. An image extraction method for extracting image data of an object from a first image that records both a background and the object to be extracted, using a mask, comprising: the first step of generating an initial mask for extracting an image of the object on the basis of difference data between the first image and a second image that records the background alone; the second step of growing a region of the generated initial mask on the basis of a similarity between a feature of a first region of the first image corresponding to the initial mask, and a feature of a second region in the vicinity of the first region; and the third step of extracting the image data of the object from the first image on the basis of the grown mask region.