A numerical control device comprising an acceleration/deceleration characteristic acquisition part which acquires predetermined acceleration/deceleration characteristic information showing a correspondence between a command speed and a maximum acceleration of a spindle, a rotation amount acquisition part which acquires predetermined rotation amount information showing the rotation amount of the spindle for a period from when feed operation of the spindle is started to when a tapping tool reaches a bottom of the hole of a workpiece, a machining time calculation part which calculates a correspondence between the command speed and a machining time required to reach the rotation amount, based on the acceleration/deceleration characteristic information and the rotation amount information, and a speed determination part which determines an optimum value of the command speed for minimizing the machining time, based on the correspondence calculated by the machining time calculation part.