A Faraday rotator of an opto-isolator in a laser processing apparatus includes a crystal cylinder that causes the Faraday effect, an enclosing tube, cooling tubes, and a magnet. The enclosing tube encases the crystal cylinder. The cooling tubes are sandwiched between the crystal cylinder and the enclosing tube at both ends of the crystal cylinder. The cooling tubes have passages through which a coolant flows. The coolant circulates through a space between the crystal cylinder and the enclosing tube, and the passages, thereby cooling the crystal cylinder.