To provide means for adding an ACD function without buying a new telephone exchange system.

A telephone number conversion adapter inserted between a telephone exchange device accommodating a plurality of extension telephone machines and outside line, has: called number extraction means extracting a called number; telephone number storage means preliminarily storing the called number and individual calling telephone numbers for individually calling the respective extension telephone machines accommodated in the telephone exchange device; and incoming call relay means converting the called number extracted by the called number extraction means into a predetermined telephone number according to a preliminarily-defined conversion rule, or alternatively relaying a signal related to the incoming call to the telephone exchange device without conversion. In the case that the called number extracted at call incoming is accorded with contents stored in the telephone number storage means, the called number is converted into a predetermined individual calling telephone number, and the signal related to the incoming call is relayed to the telephone exchange device.