To provide a bag having a flat bottom and made of a flexible thin sheetlike material, especially a plastic material, and a process for making the bag.

A folded leaf comprising a first main flap and a second main flap, which overlap with each other, and a bottom edge joining the main flaps is prepared. The folded leaf is tucked in at the bottom edge to form bellows including a first and a second base strip inserted between the main flaps and short furrows, which are incised and welded and are spaced from each other, are made at the bellows. A base strip is turned up on the main flap to form triangular pockets at the short ferrows and the triangular pockets are fixed to the base strips and each bag is separated from the folded leaf.