A vibration suppressing device includes a memory unit storing a stable rotation speed calculated by an arithmetical unit in association with a rotation speed range that includes the stable rotation speed, in which, when a command rotation speed that is a rotation speed of a rotary shaft is inputted in starting machining operation by rotation of the rotary shaft and, the arithmetical unit determines whether the command rotation speed is included in the rotation speed range or not, reads out the stable rotation speed when the command rotation speed is included in the rotation speed range and outputs the stable rotation speed to an NC device instead of the command rotation speed, and starts the machining operation at the stable rotation speed.