1. A format conversion interface unit adapted for connection to numerical control equipment which numerically controls a machine tool by interpreting and executing an NC cutting program through use of an operation function of an NC processor, comprising: a receiving circuit, coupled to the NC processor, for receiving a macro instruction delivered from the NC processor; format converting means, separate from the NC processor, for converting the macro instruction received by said receiving circuit into a series of micro instructions in accordance with a conversion program stored in said format converting means, so that the NC cutting program can be used to cause said format converting means to generate different micro instructions by rewriting only the conversion program stored in said format converting means; and a transmitting circuit, coupled to the NC processor and said format converting means, for transmitting the series of micro instructions obtained by said format converting means to the NC processor.