A portable telephone 1 is a portable terminal device in which a telephone receiving housing 2 is coupled to a telephone transmitting housing 3 by a coupling portion 4 such that they can be opened and closed. In the telephone receiving housing 2, a projecting portion 21 is formed on an end portion of a first face S1, which faces the telephone transmitting housing 3 in the closed state. In the telephone transmitting housing 3, a notched portion 22 is formed in an end portion thereof, in which the projecting portion 21 is arranged such that the projecting portion is rotatably coupled to notched portion 22. A switch is provided inside the projecting portion 21 and a sub-key capable of depressing this switch is provided to the projecting portion 21. The sub-key is provided on a surface of the projecting portion 21 in such a manner that a depression direction of the sub-key coincides with a depression direction of the main key in the opened state.