Provided is a computer implemented method for performing mutual authentication between an online service server and a service user, including: (a) generating, by an authentication server, a server inspection OTP; (b) generating, by an OTP generator, a verification OTP having the same condition as the server inspection OTP and using the same generation key as an OTP generation key and a calculation condition different from a calculation condition is applied or a generation key different from the OTP generation key is used and the same calculation condition as the calculation condition used for generating the server inspection OTP is applied to generate a user OTP; and (c) generating, by the authentication server, a corresponding OTP having the same condition as the user OTP and comparing whether the generated corresponding OTP and the user OTP match each other to authenticate the service user.