A machine learning device includes: a state observer that observes, as a state variable, at least one of driving noise of a synchronous rotating machine and an error with respect to a preset position of a rotational position of the synchronous rotating machine determined by driving voltage; and a learner that determines a value of a parameter on a basis of the state variable observed by the state observer. The learner includes: a reward calculator that calculates a reward on a basis of the state variable; and a function updater that determines the value of the parameter on a basis of the reward that has been calculated. The reward calculator increases the reward when the driving noise is smaller than a target value of driving noise.