Data is compressed/encrypted for communication between application programs without addition of any modification to the application programs. In a communication system where a plurality of personal computers are connected to each other using communication lines, a personal computer is provided with a communication program and an application program, the application program uses a command offered by the communication program for transmission and reception of data with another personal computer, a hooking program for discriminating the command related to the data communication issued by the application program is provided and, after the hooking program has compressed or encrypted the data of application program depending on a type of the command, the command is transferred to the communication program.