Abstract: A disclosed substrate processing method includes steps of: arranging a substrate in a chamber; introducing H2 gas at a first flow rate and O2 gas at a second flow rate independently from the H2 gas into a catalyst reaction portion in which catalyst is accommodated, wherein H2O gas produced from the H2 gas and the O2 gas that contact the catalyst is ejected from the catalyst reaction portion toward the substrate; and reducing a flow rate of the O2 gas introduced to the catalyst reaction portion to a third flow rate that is lower than the second flow rate, wherein the steps of introducing the H2 gas and the O2 gas and reducing the flow rate of the O2 gas are repeated in this order at a predetermined repetition frequency, thereby processing the substrate.