A storage means access control method for an NC system, which has no adverse effect on the execution of an NC instruction. The NC system has a first storage means and a second storage means having an access time longer than that of the first storage means, and sequentially executes NC instructions in blocks of an NC program stored in the first storage means, block by block. The NC instructions in the blocks of the NC program are previously classified into waitable instructions and unwaitable instructions. When there is a request for access to the second storage means during execution of the NC program, if an NC instruction in an execution block to be executed subsequently is a waitable instruction, access to the second storage means is executed after the NC instruction in the execution block has been executed. If the NC instruction in the execution block is an unwaitable instruction, access to the second storage means is not executed after the NC instruction in the execution block has been executed.