A substrate bonding method for bonding two substrates includes an activation treatment step of, by subjecting at least one of bonding surfaces to be bonded to each other of respective ones of the two substrates to at least one of reactive ion etching using nitrogen gas and irradiation of nitrogen radicals, activating the bonding surface, a gas exposure step of, after the activation treatment step, exposing the bonding surfaces of the two substrates to gas containing water within a preset standard time, and a bonding step of bonding the two substrates that have the bonding surfaces activated in the activation treatment step.