It is possible to prevent unauthorized use of software such as a game program in download type software distribution for using a program and data by storing them in a user computer in advance. A user downloads a game program of new version (including an accompanying data group) and an execution program from a distribution computer. The execution control program performs authentication by online with an authentication computer and acquires start information required for starting the game program. The execution control program decodes the game program according to the start information and starts it. When the user terminates the game by terminating the game program, the execution control program deletes all or part of the game program so as to incapacitate it.