A steam oxidation apparatus is provided which is capable of ensuring a desirable controllability and reproducibility of the steam oxidation of an object-to-be-oxidized housed in the reactor, by suppressing condensation of the steam in the steam-accompanied inert gas supplied to the reactor. The steam oxidation apparatus 78 is an apparatus used for forming the current confinement structure into the surface-emitting laser element by subjecting the high-Al-content layer to steam oxidation, and is equipped with a reactor 42 for the steam oxidation, a steam-accompanied inert gas system for supplying a steam-accompanied inert gas to the reactor 42, an inert gas system for supplying an inert gas to the reactor 42, a reactor bypass pipe 52 for allowing the steam-accompanied inert gas system and inert gas system to bypass the reactor, and an exhaust system for discharging exhaust gas from the reactor 42. The steam oxidation apparatus 78 is further equipped with a thermostatic oven 72 which houses the H2O bubbler 60, second gas pipe 68, automatic open/close valves 66A to 66D, a portion of the third gas pipe 70 in the vicinity of the automatic open/close valves 66A to 66D, and a portion of the gas inlet port 42A side of the reactor 42.