A method of authenticating users by using one time password and a system thereof

PURPOSE: A method and a system for certifying a user using an OTP(One-Time Password) through a different communication network are provided to maximize user's convenience by receiving the OTP through the mobile communication network and using the OTP to user certification without installing an OTP generation application to a terminal. CONSTITUTION: The first network interface(640) transmits/receives data with the first user terminal or a specified service system through the first network. The second network interface(650) transmits/receives the data with the second user terminal through the second network. A challenge code generator(610) generates a challenge code by responding to a request from the service system. A response code generator(620) generates a response code for the challenge code. An SMS(Short Message Service) server(680) certifies the user by comparing the generated response code with the response code received from the first terminal through the first network and transmits a short text message to the second user terminal through the second network.