To provide a bag mouth folding method which enables folded portions of a folded surface of a bag mouth of a package to be converged to a binding part, and deploying the bag mouth of the package in a fan shape with a new position of the binding part being a pivot of the fan.

The bag mouth folding method comprises steps of: repeating the ridge folding and the valley folding from both ends to a binding part along an auxiliary line section when specifying the auxiliary line section substantially orthogonal to upper and lower center line sections including the binding part while the binding part is in the position of a predetermined distance from a substantially center upper part to a substantially center lower part of the package; converging the folded parts formed by repeating the ridge folding and the valley folding of the folded surface to the binding part; binding the folded part of the package converged on the binding part; and pulling out a center upper part of the bag mouth of the bound package.