Abstract: A device for converting a flat pouch into a tetrahedral pouch in a rotary packaging machine including: grippers having gripper jaws and gripper rollers; a rotary shaft; and a barrel cam combined with the rotary shaft and reciprocating for a duration as long as one process of the grippers, with a cam portion provided on an upper part of the barrel cam to control the space between the gripper jaws; the device including: an arm unit combined with the rotary shaft and performing repeated reciprocation for a duration as long as one process of the rotary packaging machine; a rotary unit mounted to the arm unit; and a pouch clamping unit combined with the rotary unit to be rotated by the rotary unit, the pouch clamping unit clamping opposite sides of the mouth of the pouch while opening the mouth of the pouch inside the pouch.