1. An image processing device comprising: a processor; and a memory which stores a plurality of instructions, which when executed by the processor, cause the processor to execute: obtaining a first image and a second image; determining a first direction of a certain first reference point of a first region of a user included in the first image and a second direction of a certain second reference point of a second region of the user included in the second image; extracting a first feature quantity from the first region and a second feature quantity from the second region; generating a third feature quantity from the first feature quantity or a fourth feature quantity from the second feature quantity on the basis of the first direction or the second direction; and authenticating the user on the basis of similarity between the first feature quantity or the third feature quantity and the second feature quantity or the fourth feature quantity.