An end of a sheet drawn out form a feeder is clamped by a pair of upper and lower clamping devices of two or more pairs. The sheet is drawn out from the initial position to the end position in a spread state. While the spread sheet is being tentered on pin tenters, another pair of upper and lower clamping devices at the end position is separated from each other and moved along the respective rails into the initial position. The end of the sheet drawn out from the feeder is clamped from above and below at the initial position with the upper and lower clamping devices that has returned to the initial position.