A transmission control system comprises a restricted word database for storing restricted words; a request transmission control section for relaying a request message sent from a Web browser to an external network and inspecting whether the request message includes a code that is dependent on an HTML file; and a restricted word checking section for analyzing the code using related portions of the code described in the HTML file when the request message includes the code, and determining whether or not to allow transmission of the request message depending on whether an analysis result of the code corresponds to the restricted words, wherein the request transmission control section controls the transmission of the request message according to the determination of the restricted word checking section.