Provided are a method and a device for processing a network slice-specific authentication and authorization (NSSAA) failure caused by a network error or timeout. An access and mobility management function (AMF): starts an NSSAA procedure for an NSSAA function (NSSAAF) and single network slice selection assistance information (S-NSSAI); and restarts the NSSAA procedure after waiting for a certain period, on the basis that the Hypertext Transfer protocol (HTTP) status code “504 Gateway Timeout” is received from the NSSAAF, and/or a response regarding the NSSAA procedure could not be received from the NSSAAF.