A plastic bag comprising: two sheet panels opposing each other to have opposite side edges; a pair of side gussets extending along the opposite side edges, each of the side gussets being folded in half and interposed between the sheet panels to have a folded inner edge, open outer edges, opposite end edges, and opposite end portions, each of the side gussets further including a flap between two layers thereof, the flap being formed by one of the opposite end portions which is obliquely folded in half along a fold line and interposed between the two layers; and two first heat sealed parts formed by means of heat sealing the sheet panels to the side gussets along the opposite side edges of the sheet panels, the flap including: a folded edge formed by the folded inner edge; an open edge formed by one of the opposite end edges and aligned with the open outer edges; a second heat sealed part formed by means of heat sealing two layers of the flap to each other along the open edge; and a joining edge extending straight or curved and joining the folded edge and the open edge on opposite side of the fold line, the first heat sealed parts including: main sections extending along the opposite side edges of the sheet panels; and protruding sections protruding from the main sections toward an inside of the plastic bag, the second heat sealed part including: a base section extending along the open edge; and a complementary section extending from the base section to the folded edge, wherein the open edge and the joining edge are included in the second heat sealed part, and wherein the complementary section and the protruding sections are formed opposing each other. two sheet panels opposing each other to have opposite side edges; a pair of side gussets extending along the opposite side edges, each of the side gussets being folded in half and interposed between the sheet panels to have a folded inner edge, open outer edges, opposite end edges, and opposite end portions, each of the side gussets further including a flap between two layers thereof, the flap being formed by one of the opposite end portions which is obliquely folded in half along a fold line and interposed between the two layers; and two first heat sealed parts formed by means of heat sealing the sheet panels to the side gussets along the opposite side edges of the sheet panels, the flap including: a folded edge formed by the folded inner edge; an open edge formed by one of the opposite end edges and aligned with the open outer edges; a second heat sealed part formed by means of heat sealing two layers of the flap to each other along the open edge; and a joining edge extending straight or curved and joining the folded edge and the open edge on opposite side of the fold line, the first heat sealed parts including: main sections extending along the opposite side edges of the sheet panels; and protruding sections protruding from the main sections toward an inside of the plastic bag, the second heat sealed part including: a base section extending along the open edge; and a complementary section extending from the base section to the folded edge, wherein the open edge and the joining edge are included in the second heat sealed part, and wherein the complementary section and the protruding sections are formed opposing each other. a folded edge formed by the folded inner edge; an open edge formed by one of the opposite end edges and aligned with the open outer edges; a second heat sealed part formed by means of heat sealing two layers of the flap to each other along the open edge; and a joining edge extending straight or curved and joining the folded edge and the open edge on opposite side of the fold line, the first heat sealed parts including: main sections extending along the opposite side edges of the sheet panels; and protruding sections protruding from the main sections toward an inside of the plastic bag, the second heat sealed part including: a base section extending along the open edge; and a complementary section extending from the base section to the folded edge, wherein the open edge and the joining edge are included in the second heat sealed part, and wherein the complementary section and the protruding sections are formed opposing each other. main sections extending along the opposite side edges of the sheet panels; and protruding sections protruding from the main sections toward an inside of the plastic bag, the second heat sealed part including: a base section extending along the open edge; and a complementary section extending from the base section to the folded edge, wherein the open edge and the joining edge are included in the second heat sealed part, and wherein the complementary section and the protruding sections are formed opposing each other. a base section extending along the open edge; and a complementary section extending from the base section to the folded edge, wherein the open edge and the joining edge are included in the second heat sealed part, and wherein the complementary section and the protruding sections are formed opposing each other.