Provided is a numerical controller capable of efficient signal transmission and reception to and from a retrofitted PLC. A numerical controller includes a numerical control unit, a built-in PLC, and a retrofitted PLC operating at a predetermined control period different from those of the numerical control unit and the built-in PLC. The retrofitted PLC is configured to detect external triggers issued from the numerical control unit and the built-in PLC, execute a sequence processing for numerical control processing upon detection of the external trigger issued from the numerical control unit, and execute a sequence processing for built-in PLC processing upon detection of the external trigger issued from the built-in PLC.