A portable terminal device in which a case body performs sliding movement has had a problem that a groove may be exposed to the exterior when the case body is opened to a full extent of sliding amount, which will deteriorate grade of the portable terminal device and sealing ability of the case body, permitting dust or the like to enter the cases. In the portable terminal device according to the present invention in which a front side case is adapted to slide with respect to a back side case, a protruding member in a substantially T-shape in cross section is provided on the front side case, and a guide groove frame is provided on the back side case so as to enclose the protruding member, and a track along which the protruding member is adapted to move is defined in the guide groove frame. When the protruding member is positioned at both ends of the guide groove frame, the protruding member and the guide groove frame are arranged in such positional relation that an entire groove region of the guide groove frame is within an opposed range of the front side case. According to this structure, the groove will not be exposed to the exterior in either state where the case body is closed or opened by sliding, intrusion of dust or the like into the groove will be prevented, and sealing ability of the case body can be improved.