Abstract: A method for updating software in a radio terminal device of a mobile communication system, wherein a base station and radio terminal devices are connected mutually through radio communication channels, including the steps of notifying version information on a control-software of the radio terminal device to a software-supply device connected to a network by the radio terminal device, determining a necessity of updating the control-software by comparing the version information received from the radio terminal device with latest version information stored in and managed by the software-supply device, and downloading new control-software that is appropriate to update the version of the control-software to the radio terminal device by the software-supply device if updating of the control-software is needed.