A gusset bag is formed of a pair of flat surface parts facing each other, and side gusset parts folded to the inside of the bag. The gusset bag has side heat-sealing portions at which the sides of the flat surface part and sides of the side gusset parts are joined at the inner surfaces thereof, side heat-sealing portions at which the sides of the flat surface part and the other sides of the side gusset parts are joined at the inner surfaces thereof, and closing sealing portions at which the side heat-sealing portions and the side heat-sealing portions are joined at the outer surfaces of the side gusset parts by ultrasonic sealing. With this configuration, in the gusset bag, it is possible to easily close the gusset parts, while preventing film pieces or dust from entering the bag in a step of closing the gusset parts.