1. A method of revising NC programs for a four-axis lathe in which there are provided first and second NC programs corresponding to first and second tool rests, respectively, the first and second NC programs each having wait commands, wherein a workpiece is machined by controlling movement of the first and second tool rests independently in accordance with the first and second NC programs, respectively, while synchronization is achieved by the wait commands, said method comprising the steps of: (a) executing interpolative simulations one block at a time in successive fashion based on the first and second NC programs when an interference check is made; (b) checking whether tools on the first and second tool rests interfere with each other during said step (a); (c) when it is determined in said step (b) that there is interference, halting the interpolative simulations and moving the two tool rests back to the beginning of blocks in which the interference occurred; (d) holding the interpolative simulation according to one of the first and second NC programs in the halted state and executing the interpolative simulation according to the other of the first and second NC programs to move solely the corresponding tool rest along a path in the block in which the interference occurred while checking for interference; (e) when no interference is found when moving solely the one tool rest in said step (d), storing sequence numbers of the first and second programs which prevailed when interference occurred; and (f) automatically revising the NC programs after the interference check by inserting a wait command ahead of the sequence number of the NC program for which the interpolative simulation was halted in said step (d), and inserting a wait command after the sequence number of the NC program for which the interpolative simulation was executed in said step (d).