To provide a packaging bag inhibited from an accidental unsealing upon a shock.

A side edge bonded part is formed by mutually heat welding a bottom surface resin sheet 31 and a pair of sidewall resin sheets 32 in a side edge bonding step. An easy unseal part is formed by making a plurality of cuts in the side edge bonded part of the sidewall resin sheet 32 in an easy unseal part forming step after the side edge bonding step. The easy unseal part of the sidewall resin sheet 32 is heated in a heating step after the easy unseal part forming step. The easy unseal part is heated to a temperature not lower than the glass transition temperature of a heat welded layer in the sidewall resin sheet 32, and the cross section of the cut in the easy unseal part is loosely welded to make the cut into a vestige in the heating step.