Machining step determination method for automatic programming, in which NC data for a lathe is automatically programmed. Machining steps are previously stored in a memory in the machining sequence. After a blank configuration (BP) and a part configuration (PF) have been input, the stored machining steps are read out of the memory in sequence. Every time a machining step is read out, a judgement is made as to whether or not the machining step is needed to obtain the final part, on the basis of the blank configuration and the part configuration. If the machining step is not needed, the subsequent machining step is read out of the memory and is subjected to a similar judgement. On the other hand, if the machining step is needed, cutting ranges (A1, A2, A3) and cutting directions for that machining step are determined, and the subsequent machining step is then read out of the memory and is subjected to similar processing, to prepare NC data.