In the solid-state laser apparatus 1, a main pipe 11 for circulating a coolant through a solid-state laser medium 3 is provided with a heat exchanger 14, whereby the laser medium 3 is prevented from raising its temperature. When the coolant becomes acidic or alkaline, a controller 24 controls a flow regulating valve 23, so as to increase the flow rate of the coolant flowing into a bypass pipe 21 provided with a deionizing filter 22, whereby the acidity or alkalinity of the coolant can be weakened. This can prevent the coolant from deteriorating a predetermined part of the laser apparatus 3, and thus can improve the durability of the solid-state laser apparatus 1. Also, since the bypass pipe 21 connected in parallel to a part of the main pipe 11 is provided with the deionizing filter 22, the flow rate of the coolant circulating through the main pipe 11 can be restrained from decreasing, and the cooling efficiency of the laser medium 3 can be prevented from lowering.