To provide a form gathering machine for a DP bag or the like improving the efficiency of a process of forming a bag mouth to facilitate taking-in/out, while improving the quality.

This form gathering machine stacks and sticks a plurality of slip sheets 3-5 attached with pin holes (a) for feeding sheets in their one side to a bag lower sheet 1 and a bag upper sheet 2 attached with the pin holes (a) for feeding the sheets in their both sides, gathers them using the pin holes as the references, and removes the marginal regions having the pin holes in the respective sheets as unnecessary parts 6 excluding the bag lower sheet 1. This form gathering machine which is provided with a slitter part comprising a thin-sheet lower blade 10 having a cutting edge in one side and a disk upper blade 20 performing the cutting operation by shearing between itself and the cutting edge of the lower blade 10, cuts and removes the unnecessary parts while carrying the sheet whole bodies with the lower blade inserted between the bag lower sheet 1 and the bag upper sheet 2.