A controller for controlling a machine includes a coolant monitoring function. The controller generates and records history information in which the measurement values of a state of a coolant are associated with time information on the time when the measurement values are measured. Then, a rate of change in a measurement value for a state of the coolant is calculated based on the recorded history information, and a state of the coolant and a countermeasure therefore are determined based on the calculated rate of change in the measurement value.