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 with a bonding material 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 by connecting an exhaust device to the evacuation port and driving the exhaust device. The sealing step includes sealing the evacuation port with the sealing material while an evacuated state in the internal space is maintained. In the sealing step, a measured value by a pressure gauge is monitored while the sealing material is heated, softening of the sealing material is detected based on the transition of the measured value, and heating of the sealing material is stopped.