An object would be to propose a glass panel unit which does not have an outlet and an evacuation pipe but has improved thermal insulating properties. The method includes: an assembling step of preparing a temporary assembly ( 100 ) including: a first glass substrate ( 200 ) and a second glass substrate ( 300 ); a frame ( 410 ) placed therebetween; an inside space ( 500 ) enclosed by the first glass substrate ( 200 ), the second glass substrate ( 300 ), and the frame ( 410 ); a partition ( 420 ) dividing the inside space ( 500 ) into a first space ( 510 ) and a second space ( 520 ); a gas passage ( 600 ) interconnecting the first space ( 510 ) and the second space ( 520 ); and an outlet ( 700 ) interconnecting the second space ( 520 ) and an outside space; a hermetically enclosing step of obtaining a completed assembly ( 110 ) by: converting the first space ( 510 ) into an evacuated space ( 50 ) by evacuating the first space ( 510 ) through the gas passage ( 600 ), the second space ( 520 ), and the outlet ( 700 ) and changing a shape of the partition ( 420 ) to close the gas passage ( 600 ) to form a separator ( 42 ) to form a seal ( 40 ) enclosing the evacuated space ( 50 ); and a removing step of removing part including the second space ( 520 ) from the completed assembly ( 110 ) by cutting the completed assembly ( 110 ) along a straight line to obtain a glass panel unit ( 10 ) which is a predetermined part including the evacuated space ( 50 ).