A method of controlling a mobile device (100, 400, 500, 700, 900, 1000, 1200, 1300, 1400) which displays a plurality of windows executing corresponding applications on a touch screen of the mobile device, the method comprising: displaying a first window and a second window on a touch screen of the mobile device, each of the first window and the second window displaying an execution screen (1212, 1222) of a corresponding application in an unlock state; receiving a lock command via the first window and the second window; and in response to receiving the lock command, changing the first window and the second window from the unlock state to a lock state, wherein changing the first window and the second window from the unlock state to the lock state comprises changing the execution screen to a lock screen (1214, 1224); wherein receiving the lock command includes receiving an input of simultaneous touches on a first lock control button of the first window and a second lock control button of the second window, or simultaneous downward drag gestures on a title bar of the first window and a title bar of the second window, as a collective lock command for both the first window and the second window; the method further comprising: in response to detecting an unlock command on the first window, displaying, on the first window, a password input screen including an interface to unlock the first window and second window by inputting a password, while the second window displays the lock screen; obtaining, via the interface included in the password input screen, the password for unlocking the first window; and displaying, on the first window and the second window, an execution screen (1212, 1222) of the corresponding application, when the password corresponds to a predetermined password. displaying a first window and a second window on a touch screen of the mobile device, each of the first window and the second window displaying an execution screen (1212, 1222) of a corresponding application in an unlock state; receiving a lock command via the first window and the second window; and in response to receiving the lock command, changing the first window and the second window from the unlock state to a lock state, wherein changing the first window and the second window from the unlock state to the lock state comprises changing the execution screen to a lock screen (1214, 1224); wherein receiving the lock command includes receiving an input of simultaneous touches on a first lock control button of the first window and a second lock control button of the second window, or simultaneous downward drag gestures on a title bar of the first window and a title bar of the second window, as a collective lock command for both the first window and the second window; the method further comprising: in response to detecting an unlock command on the first window, displaying, on the first window, a password input screen including an interface to unlock the first window and second window by inputting a password, while the second window displays the lock screen; obtaining, via the interface included in the password input screen, the password for unlocking the first window; and displaying, on the first window and the second window, an execution screen (1212, 1222) of the corresponding application, when the password corresponds to a predetermined password. in response to detecting an unlock command on the first window, displaying, on the first window, a password input screen including an interface to unlock the first window and second window by inputting a password, while the second window displays the lock screen; obtaining, via the interface included in the password input screen, the password for unlocking the first window; and displaying, on the first window and the second window, an execution screen (1212, 1222) of the corresponding application, when the password corresponds to a predetermined password.