1. A direct numerical control (DNC) operation system, comprising: a host apparatus comprising a storage device; a numerical controller comprising a storage device; and a communication line connecting the numerical controller to the host apparatus, the communication line being configured to perform bidirectional communication and to successively transmit numerical control (NC) data stored in the storage device of the host apparatus to the numerical controller to perform a DNC operation, wherein the numerical controller is configured to perform operation using the NC data received from the host apparatus via the communication line, and the numerical controller comprises a processor configured as: a determination unit configured to determine whether the NC data received from the host apparatus includes any of a sub program, a repeat instruction and a branch instruction; and a search information write processing unit configured to write, in the storage device in the numerical controller, search information for searching the NC data stored in the storage device in the host apparatus and to discard the received NC data, in response to a determination by the determination unit that the received NC data includes any of the sub program, the repeat instruction and the branch instruction, the host apparatus comprises a processor configured as: a search information read processing unit configured to read the written search information; an NC data search processing unit configured to search the NC data stored in the storage device in the host apparatus on the basis of the read search information; and an NC data transmission processing unit configured to transmit the searched NC data to the numerical controller, the numerical controller is further configured to determine whether the NC data received after the search information is written is the searched NC data that has been searched by the host apparatus, and in response to a determination that the NC data received after the search information is written is the searched NC data, perform the operation on the basis of the NC data received after the search information is written, the numerical controller is further configured to perform a prefetch antecedent to the operation of the NC data received from the host apparatus via the communication line, analyze the received NC data, and then store the analyzed NC data in the storage device of the numerical controller; and in response to a determination that the analyzed NC data includes any of the sub program call, the repeat instruction and the branch instruction, the search information write processing unit is configured to write, in the storage device in the numerical controller, search information that is required for searching for the NC data which is required by the host apparatus to execute the relevant instruction.