PURPOSE: To make sealed states of individual bags uniform and complete even if the bags contain contents not constant in length.

CONSTITUTION: An apparatus for folding bag mouth is moved by a moving member 20 from a waiting position to a working position. When an expanding member 30 is at the working position, four expanding levers 33 of the member 30 are inserted into an opening 13 of a bag. An expansion driving mechanism 40 opens the opening 13 outward by the respective expansion levers 33. Two folding levers of a folding member 50 fold a center position at each of two sides inward. Two bars of an interposing member interpose the opening 13 therebetween from outside by an interposing driving mechanism.