To prevent a power supply from being covered with dew for circuit protection and to downsize or simplify a laser apparatus: An excitation lamp 10 and a YAG rod (laser medium) 12 of a laser oscillator are disposed within a chamber 14. Heat-generating electrical components or elements of the power supply, such as diodes D1-D6 of a three-phase full-wave rectifier circuit 24, IGBT 26, GTR 30 and output transistors of driver circuits 34 and 36 are mounted on a heat sink 46. A water-cooled cooling apparatus 50 supplies deionized water (cooling water) DW whose temperature is controlled at a predetermined temperature, for instance, 25-35 degrees centigrade to the heat sink 46 of the power supply via pipes 72 and 76 as well as the chamber 14 of the laser oscillator via pipes 70 and 74.
