A server for classifying a plurality of mental states of a user is provided. The server comprised: a service platform; and a mental state classification platform. The service platform is configured to: provide a questionnaire corresponding to each of the plurality of mental states to a terminal of a user, and receive an answer of the user to the questionnaire from the terminal; receive a face image generated by photographing a face of the user while the user inputs the answer to the questionnaire for each of the plurality of mental states in the terminal of the user; by the terminal, display the face image including at least a middle of a forehead and both cheeks of the face of the user on the user interface so that the user can recognize his/her appearance while inputting the answer to the questionnaire through the user interface of the terminal; and transmit the answer received from the user and the face image to the mental state classification platform.