A glass panel unit manufacturing method includes a bonding step, a pressure reducing step, and a sealing step. The bonding step includes bonding together a first substrate and a second substrate with a first sealant to create an inner space. The pressure reducing step includes producing a reduced pressure in the inner space through an exhaust port that the first substrate has. The sealing step includes melting and expanding a second sealant, inserted into the exhaust port, by heating the second sealant and thereby sealing the exhaust port up with the second sealant expanded to the point of coming into contact with a dam arranged in the inner space.