To provide a telephone system which efficiently adds related telephone numbers of each registrant registered in a telephone directory, in the telephone directory.

When a call from a phonebook to a telephone number registered in a telephone number/related telephone number accumulation part 2 is disconnected, and the number is dialed, and when the telephone number of the dial call is not registered as the related telephone number, a related telephone number registration processing part 3 registers the dialed telephone number in association with the telephone number used for the call from the phonebook, into the telephone number/related telephone number accumulation part 2.