A superimposing control method using a numerical control device, capable of starting and terminating a superimposing control even during movement, without the need of waiting for the adjustment of timing and without rapid acceleration or deceleration. In response to a superimposition command, a motion value distributed to a Z axis of a superimposing axis in a reference system is subjected to acceleration/deceleration processing by an acceleration/deceleration processing section separately provided for superimposition, and the obtained movement value is added to a motion command value for a Z axis of a superimposed axis in a superimposing system. When a superimposition termination command is issued, the motion command value remained in the acceleration/deceleration processing section for superimposition is superimposed on the motion command value for the Z axis of the superimposing system. The superimposition can be smoothly started and terminated without the need to stop the two control systems.