A client computer that is connectable to a host computer by a network, includes a communication part to communicate with the host computer; a user input part; a system part to perform a function depending on an application; and a controller to control the system part to be put into a locking state to stop performing operations input by a user from the user input part if a locking signal is received from the host computer through the communication part, and to control the communication part to unlock the locking state if an unlocking signal is received from the host computer through the communication part.