a module is placed inside a chamber that can be sealed hermetically . the module includes a substrate incorporating a plurality of electronic circuit parts , a frame connected to the substrate so as to enclose the plurality of electronic circuit parts , and a cover placed facing the substrate and connected to the frame so as to cover an opening portion of the frame . the pressure in the chamber is reduced to a first pressure p1 , and then the pressure is increased from the first pressure p1 to a second pressure p2 . a liquid is filled into the module through a hole formed in the cover within the chamber , and the pressure inside the chamber is increased from the second pressure p2 to a third pressure p3 . then , the hole of the cover in the chamber is closed to seal the module hermetically , thereby sealing the liquid in the module . thereafter , the pressure in the chamber is increased from the third pressure p3 to an atmospheric pressure p0 , and the module is taken out of the chamber .