for the blank clamp 16 is fixed and linked the first blank sensor 19 and the second sensor 20 are designed to detect whether the hemmed piece w1 conveyed on the conveyor belts 4a is present at the specified position or not and issue a detection signal by the operation of the cylinder 15 based on the detection signal detecting the presence of the hemmed piece w1 by both sensors 19 20 the both tongues 13 14 of the clamp 16 are closed to hold the front end part of the hemmed piece w1 and by the actuation of the air cylinder 17 the clamp 16 and the front end part of the hemmed piece w1 are lifted above the conveying surface of the conveyor belts 4a to be stopped in the state shown in fig4 besides in the holding and lifting state of the front end part as shown in fig4 the rear end part of the hemmed piece w1 is conveyed continuously by the conveyor belts 4a and when the second blank sensor 20 detects the absence of the hemmed piece w1 it indicates that the rear end part has passed as a result the signal is issued and on the basis of the detection output signal both tongues 13 14 of the clamp 16 are opened by operation of the cylinders 15 and holding of the front end part of.