In a numerical control system including a plurality of data communication apparatuses and a numerical controller that transmits and receives data to and from the data communication apparatuses, a filter circuit is removably attached to each of a connection unit of the numerical controller and connection units of the data communication apparatuses. By detecting an error information based on a data transfer error or the sign of the data transfer error, a filter circuit that needs to be replaced is specified. Thus, the noise immunity of only a required portion of the filter circuits is increased, as a result, it is possible to suppress the cost for noise reduction while increasing the noise immunity of the entire system.