1. A DNC operation system that connects a host apparatus and a numerical controller via a communication line capable of bidirectional communication and successively transmits, to the numerical controller, NC data stored in a storage device in the host apparatus in order to perform a DNC operation, with the numerical controller being configured to perform operation using the transmitted NC data, wherein the numerical controller has: means for determining whether the NC data transmitted from the host apparatus includes any of a sub program, a repeat instruction and a branch instruction; and means for writing, in a storage device in the numerical controller, search information for searching the NC data stored in the storage device in the host apparatus when the NC data includes any of the sub program, the repeat instruction and the branch instruction, wherein the host apparatus has: means for reading the written search information; means for searching the NC data stored in the storage device in the host apparatus on the basis of the read search information; and means for transmitting the searched NC data to the numerical controller, and wherein the numerical controller determines whether the NC data received after the search information is written is the NC data that has been searched by the host apparatus, and performs the operation on the basis of the received NC data.