A portable terminal device comprising: a plurality of instant message services (1, 2, 3) which are application programs that provide a user with an instant message service, each of the instant message services (1, 2, 3) being signed in by using a mutually different user ID; display means for displaying a display screen including a main display area and a sub display area provided separately from the main display area; display control means for controlling a display of the display means to display, in the main display area, the content of a service provided by an instant message service selected from among the plurality of instant message services (1, 2, 3), and to display, in the sub display area, instant message service icons corresponding to other of the non-selected instant message services; message notification means for detecting that the terminal device has received an instant message, and for notifying an occurrence of an incoming instant message; and messaging user interface linking means for judging which one of the plurality of instant message services the instant message received by the terminal device corresponds to, in response to the notification from the message notification means, and for outputting a display screen update instruction to the display control means to visually change a display of the corresponding instant message service icon in the sub display area so as to display presence of an unread message for the corresponding instant message service, when the instant message is not a message for the instant message service currently displayed and browsed in the main display area; wherein the messaging user interface linking means judges which one of the plurality of instant message services (1, 2, 3) the message corresponds to, based on the user ID included in the instant message received by the terminal device. a plurality of instant message services (1, 2, 3) which are application programs that provide a user with an instant message service, each of the instant message services (1, 2, 3) being signed in by using a mutually different user ID; display means for displaying a display screen including a main display area and a sub display area provided separately from the main display area; display control means for controlling a display of the display means to display, in the main display area, the content of a service provided by an instant message service selected from among the plurality of instant message services (1, 2, 3), and to display, in the sub display area, instant message service icons corresponding to other of the non-selected instant message services; message notification means for detecting that the terminal device has received an instant message, and for notifying an occurrence of an incoming instant message; and messaging user interface linking means for judging which one of the plurality of instant message services the instant message received by the terminal device corresponds to, in response to the notification from the message notification means, and for outputting a display screen update instruction to the display control means to visually change a display of the corresponding instant message service icon in the sub display area so as to display presence of an unread message for the corresponding instant message service, when the instant message is not a message for the instant message service currently displayed and browsed in the main display area; wherein the messaging user interface linking means judges which one of the plurality of instant message services (1, 2, 3) the message corresponds to, based on the user ID included in the instant message received by the terminal device.