In a mobile terminal device, at least one heat conduction layer formed of a member, such as copper, aluminum or carbon, being excellent in heat conductivity is provided inside a circuit board on which electronic components are mounted. The heat generated in the electronic components is promptly dispersed in the direction of the face of the circuit board by the heat conduction layer, and transferred from the whole face of the circuit board to the operation member, such as keys, and the housing, and then radiated to the outside. With this structure, the local temperature rise at the operation member and the housing can be suppressed, and the temperature on the surface of the mobile terminal device can be made uniform, without significantly increasing the cost and the thickness of the mobile terminal device. In addition, high-performance electronic components can be used by adopting this structure. Furthermore, the rigidity of the circuit board can be raised, and the reliability of the mobile terminal device can be improved.