We propose the PBSM(Protocol-Based Security Module) system which guarantees the secure data transmission under web environments. There are two modules in the PBSM architecture. One is Web Server Security Module(WSSM) which is working on a web server, the other is the Winsock Client Security Module(WSCSM) which is working on a client. The WSCSM security module decrypts the encrypted HTML document that is received from the security web server. The decrypted HTML document is displayed on the screen of a client. The WSSM module contains the encryption function for HTML file and the decryption function for CGI(Common Gateway Interface). The formal analysis methodology is imported from format theory for analyzing the data flow of the PBSM system. The formal analysis methodology is based on the order theory.