Recently in some nations, the interface of NC machine tool is by applying the softwired sequence control method which employs the PLE(Programmable Logic Controller) instead of the hardwired sequence control method. Due to this replacament, the funcion of the interface of NC machine tool has been improved in many respects. In order to accomplish such as improvement of the function of the interface and to develop the PLC, this paper deals with how to apply the sofrwired sequence control method that employs microcumputer to the interface of ATC(Automatic Tool Changer) which is a part of NC lathe