To provide an antenna device and a portable telephone set in which receiving property of the antenna cannot change easily in a stand-by condition (folded condition) and communication, conversation condition (opened condition) in a folding type carrier telephone.

A feeding element (antenna element) 21 which is fed from a feeding part 17, is formed on a lower substitute 15b out of facing planes when an upper substitute 15a and the lower substitute 15b are folded. The feeding element 21, for example, is bent after extended along a side end face from an open side end part on the lower substitute 15b, then extended toward a center. By this configuration, an area of the antenna element can be large. An auxiliary element 23 which is a passive element, is arranged in an area which is an area facing to a region extending to the center out of the feeding element 21. The feeding element 21 and the passive element 23 are arranged in a facing sides when folded.