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 in which a user is included, a second image which is imaged in an imaging condition different from that of the first image, a third image which is continuously imaged at a different point of time from that of the first image, and in the same imaging condition as that of the first image, and a fourth image which is continuously imaged at a different point of time from that of the second image, and in the same imaging condition as that of the second image; extracting a first feature amount of a user which is included in the first image, a second feature amount of the user which is included in the second image, a third feature amount of the user which is included in the third image, and a fourth feature amount of the user which is included in the fourth image; continuously authenticating the user based on a comparison between the first feature amount and the third feature amount, or a comparison between the second feature amount and the fourth feature amount; and switching the comparison between the first feature amount and the third feature amount to the comparison between the second feature amount and the fourth feature amount which are performed in the authenticating by alternately obtaining the second image and the third image in the obtaining while the authentication is performed based on the comparison between the first feature amount and the third feature amount in the authenticating.