PURPOSE: To solve the problem of causing a reduction in the open area of an opening part of a synthetic resin made bag having side gussets and a fastener due to the folding pieces of the side gussets in order to obtain a large opening part across the entire width of the bag.

CONSTITUTION: A synthetic resin made bag has a fastener 3 of interlocking projection and recess type provided inside an opening part 2 and side gussets, each having attached to each side 5 of the bag a V-shaped folding piece 4 formed of the film in a single or double layer. This bag is characterized by positioning the upper edge of the folding piece 4 below the fastener 3.