Provided is a bag making machine for making a bag with an angled bottom by a system different from the conventional system. Body materials of an upper layer and a lower layer are overlapped with each other and fed in the length direction. Furthermore, a bottom gusset material is fed to the body material of one layer and laid on the outer surface thereof, and one side edge of the bottom gusset material is arranged along one side edge of the body material. After the overlap of body material of the upper and lower layers and the feed of bottom gusset material, the bottom gusset material and the body material of the other layer are bonded at the one side edges thereof and fixed temporarily. As the body material is fed thereafter, the body material of one or the other layer and the bottom gusset material are guided by a guide mechanism of body material and bottom gusset material near one side edge of the body material, and the body material of one or the other layer is bent to combine the bottom gusset material with the body material of both layers. Body portion of a plastic bag is formed of the body material of both layers and the bottom gusset portion of the plastic bag is formed of the bottom gusset material.