Manufacturing quality of a semiconductor device can be improved, and manufacturing throughput can be improved. A method of manufacturing a semiconductor device includes (a) placing a substrate on a substrate supporting unit installed in a processing chamber, the substrate having thereon a solder with an oxygen-containing film on a surface thereof, (b) reducing the oxygen-containing film by supplying a reducing gas into the processing chamber while maintaining a thermal conductivity of an inner atmosphere of the processing chamber at a first thermal conductivity, and (c) melting the solder by supplying a thermally conductive gas into the processing chamber while maintaining the thermal conductivity of the inner atmosphere of the processing chamber at a second thermal conductivity higher than the first thermal conductivity.