A method of automatically preparing numerical control data comprises: providing a multiplicity of parameters (P2000 to P2034) which are used for specifying a machining pattern, and a multiplicity of function codes for preparing numerical control data, the function codes being specified by the parameters; prestoring in a memory (105) a corresponding relationship between each function code and a numerical control data output mode; prestoring in another memory (104) a corresponding relationship between each parameter and one or more function codes specified by that parameter; successively inputting the parameters to a processor (102) from a machining pattern data output device (103) in accordance with NC machining; and preparing numerical control data by employing the function codes specified by the parameters.