A data transmission server can cause identification information of a user who sent data and identification information of the user contained in the data to correspond to each other on one-to-one basis so as to prevent a person from transmitting data by pretending to be others. A memory stores client identification information for identifying the client and user identification information for identifying a user who operates the client, the client identification information and the user identification information being stored in correspondence to each other. The data received from the client is transmitted to the destination only when the client identification information of the client which transmits the data and the user identification information of the user contained in a header part and a data part of the data are stored in the memory in correspondence to each other.