A numerical controller which can prevent the stoppage of operation caused by the shutting off of communication with a host computer includes a communication unit for sending an NC data acquisition request containing a data size specification to a host computer, and a numerical control unit for performing operation using NC data sent from the host computer in response to the NC data acquisition request. Every fixed period, the communication unit sends, to the host computer, the NC data acquisition request containing the data size specification corresponding to a quantity of the NC data consumed by the numerical control unit.