A user input apparatus is provided with user input means for the user to input data or a command by using the user's human body, and use-form detection means for detecting a form in which the user uses the user input means by the user's human body. A computer connected to the user input apparatus changes the operation of the application being executed by application execution means, according to a detection result obtained by the use-form detection means. Therefore, input operations can be performed without inconvenience even if the user cannot use one hand due to another work.