A packaging bag for secondary packaging which allows safety transport of infectious substances is provided. Front and back surface materials are placed on top of another and formed into a bag member by heat-sealing both side edge and bottom edge portions continuously. Partitioning materials formed of a plastic sheet or film having a required size are disposed on inner surfaces of the upper ends of the front and back surface materials and attached to the front and back surface materials by heat-sealing both side edge and bottom edge portions except for an upper end edge portion to form pocket portions. A sealing material is stuck to the back surface material on the outside of the upper end portion thereof so as not to be opened. The internal pressure is received by one of the pocket portions when the internal pressure in the packaging bag is increased.