Abstract: A continuous use authentication method includes: acquiring a normal state image and a current image for a user; determining an inferred classification state for the user from among a plurality of classification states by applying the normal state image and the current image to a low-depth inference model for the user; when the inferred classification state is included in an abnormality category, determining a high-depth inference model corresponding to the inferred classification state from among a plurality of high-depth inference models; and determining reliability of the inferred classification state by applying the current image to the high-depth inference model.