Provided are a controller and a machine learning device that perform machine learning to optimize the servo gain of a machine inside a facility in accordance with action conditions, action environments, and a priority factor of the machine. The control system observes machine information on a machine as state, acquires information on machining by a machine as determination data, calculates a reward based on the determination data and reward conditions, performs the machine learning of the adjustment of the servo gain of the machine, determines an action of adjustment of the servo gain of the machine based on the state data and a machine learning result of the adjustment of the servo gain of the machine, and changes the servo gain of the machine, based on the action of adjustment of the determined servo gain.