1. An image processing apparatus comprising: first extracting means for extracting first feature quantities from image data of an image of a subject captured in a first period; first estimating means for estimating a first average value of the first feature quantities on the basis of second feature quantities extracted from image data of an image of a registered subject captured in a second period, which is before the first period, and third feature quantities extracted from image data of an image of the registered subject captured in a third period, which is before the first period and which is different from the second period; second estimating means for estimating a first distribution width indicating the width of dispersion of the first feature quantities with respect to the first average value on the basis of the second feature quantities and the third feature quantities; and determining means for determining whether or not the first feature quantities are within a predetermined range based on the first average value and the first distribution width.