A substrate bonding method includes: forming first plasma on a bonding surface of a first substrate at atmospheric pressure by using a mixed gas including an inert gas and water vapor, to thereby perform surface activation treatment on the bonding surface of the first substrate; forming second plasma on a bonding surface of a second substrate at atmospheric pressure by using the mixed gas, to thereby perform surface activation treatment on the bonding surface of the second substrate; bonding the bonding surface of the first substrate and the bonding surface of the second substrate to each other; and moving each of the first substrate and the second substrate at a constant speed in a region above a linear reactor in which the first plasma and the second plasma are formed.