To provide a mobile terminal device that can eliminate influence of static electricity upon signal lines of a signal-line flexible board that is placed through a connecting unit that connects two casings to be able to overlie one another. In the mobile terminal device, one of the casings and the other casing are connected through a connecting unit to be able to overlie one another, and a signal-line flexible board that connects circuit boards housed in one of the casings and the other casing, respectively, is placed through the connecting unit. The mobile terminal device includes a pair of frame-grounded flexible boards that are formed wider than the signal-line flexible board to have a conductor pattern frame grounded, and sandwich both outer surfaces of the signal-line flexible board therebetween.