A coolant supply system includes a coolant tank for accumulating coolant generated by using deionized water, a coolant supply passage for supplying the coolant in the coolant tank to the laser processing head, an electric conductivity meter for measuring an electric conductivity of the coolant to be supplied to the laser processing head, a drainage passage capable of draining the coolant from the coolant tank, an electromagnetic valve provided on the drainage passage, and a controller that controls open/close operations of the electromagnetic valve. The controller turns the electromagnetic valve from a closed state into an open state to drain the coolant in the coolant tank through the drainage passage when a measured value of the electric conductivity meter exceeds over a preset upper limit value.