In checking tool rests for interference, interpolative simulations are executed by interpolation simulators (103, 104) one block at a time in successive fashion based on first and second NC programs (101, 102). In a case where the tool rests interfere with each other, the interpolative simulations are halted and both tool rests are moved backward to the beginning of the blocks in which the interference occurred. Thereafter, with the interpolative simulation based on one NC program being kept in the halted state, the interpolative simulation based on the other NC program is executed to move the other tool rest separately along a path in the interfering block. If interference does not occur during this separate movement, sequence numbers N, M of the firt and second NC programs in the blocks in which the interference occurred are stored in a storage area (301a). Thereafter, a similar interference check is performed. After the interference check, the first and second NC programs 101, 201 are revised by inserting a wait command ahead of the block of sequence number M in the NC program for which the interpolative simulation was halted, and inserting a wait command after the block of sequence number N in the NC program for which the interpolative simulation was executed.