To provide a heat treatment apparatus for easily removing particles in a treatment chamber, and to provide a method of washing the heat treatment apparatus.

The heat treatment apparatus lifts a susceptor 73 and a heating plate 74 before performing heat treatment of a semiconductor wafer W after completing maintenance of the apparatus, and forms a gas stream of nitrogen gas toward a discharge passage 84 from an introduction passage 78. In the state, the gas in the chamber 65 is instantaneously expanded and contracted by lighting a flash lamp 69 to disperse the particles deposited on a bottom plate 62 or the like. The dispersed particles are removed by the gas stream of the nitrogen gas discharged from the discharged passage 84 by passing the bottom of the chamber 65. The particles in the chamber 65 can be easily removed only by lighting the flash lamp 69 in a prescribed number of times at constant intervals while forming the gas stream of the nitrogen gas.