To fold the bottom material of a plastic bag by folding even the large bottom material 1 of the plastic bag double when the bottom material 1 of the plastic bag is folded double along its centerline 2 and both ends thereof are folded into both layers of the bottom material 1.

First and second holders 5a and 5b are opposed mutually in a spaced apart state and both side edges 6a and 6b of the bottom material 1 are held between the first and second holders 5a and 5b. Next, the first and second holders 5a and 5b are allowed to approach each other and rotated around both side edges 6a and 6b of the bottom material 1 to be turned symmetrically.