PURPOSE: To enable the continuous and high speed manufacture of valved sacks by a method wherein an infolding device is placed at regular intervals on a rotary frame and installed is a conveyor which delivers tubular sections to the rotary frame and discharge the tubular sections with an infolded side gusset from the rotary frame.

CONSTITUTION: A plurality of side gusset infolding devices 7 are reciprocally placed at 90° intervals around the periphery of a rotary frame 13. An endless conveyor belt 16 for a double belt conveyor 15 is hung around a pulley 17 adjoining the rotary frame 13 and delivers to the approximate tangent direction of the rotary frame 13, tubular sections 14 on which an infolding sections are mounted to form a valve. When an infolded side gusset portion to form a valve passageway is formed by side gusset infolding devices at the end of the tubular sections, the properly prepared tubular sections are received by the double belt conveyor 30. The double belt conveyor 30 discharges the tubular sections to a seam station.