A portable telephone maintenance service system and maintenance service method for backup control of a portable telephone set, to make it possible to prevent misuse by third persons and to perform data restoration service. A maintenance service center is connected to an exchange network and by a backup control request, that includes the portable telephone number and a code number from the person who lost the portable telephone set, dialing is done from the maintenance service center to the lost portable telephone set. Memory dial data, mail address data, and other data from the memory of the portable telephone set is saved to backup memory and the data in the memory of the portable telephone set is erased. Upon a backup control request when the portable telephone set is recovered, data is downloaded from the backup memory to the memory of the portable telephone set.