To provide a portable telephone set capable of receiving, decoding, and reproducing enciphered music data without largely changing the constitution of a conventional portable telephone set, and an adapter to be used for the portable telephone set.

A portable telephone set 100 is constituted of a main body 105 and an adapter 140. The adapter 140 is provided with a data terminal 120, a battery 150, and a jack 1220. The data terminal equipment 120 records the enciphered music data received by the main body 105 in a memory card 110 mounted on an attaching/detaching part 1226. Also, the data terminal equipment 120 reads and decodes the enciphered music data from the memory card 110 by using a public key decoding part 1206, a common key enciphering and decoding part 1230, and a DES 1214. Then, the data terminal equipment 120 defrosts and reproduces the music data by an MP3 decoder 1216, and outputs the music data to a jack 1220.