A glass panel unit manufacturing method includes a bonding step, an insertion step, an evacuation step, and a sealing step. The bonding step includes bonding a first substrate having an evacuation port and a second substrate together with a bonding material provided between the first substrate and the second substrate and having a frame shape to form an internal space. The insertion step includes inserting a sealing material into the evacuation port. The evacuation step includes evacuating the internal space through the exhaust passage. The sealing step includes deforming the sealing material by heating while an evacuated state in the internal space is maintained. In a state where the sealing material blocks ventilation between the evacuation port and the internal space, gas is supplied through the exhaust passage toward the evacuation port.