An iris-based authentication method is provided. The method includes emitting light of an infrared wavelength band and obtaining an image based on the light of the infrared wavelength band, determining whether a specified condition is satisfied, if the specified condition is satisfied, performing user authentication (e.g., complex authentication) based on at least part of a face image and an iris image of the image that a biometric sensor obtains, or, if the specified condition is not satisfied, performing the user authentication (e.g., iris-only authentication) based on the iris image in the image that the biometric sensor obtains.