a device for centering plastic webs automatically fed thereon one by one so as to feed it to a pair of cramp chains to be correctly gripped thereby at the opposite side edges . this comprises a bed consisting of a central frame and longitidinal as well as transverse arms respectively extended in opposite directions from the central frame for supporting the web of varied size . a plurality of pairs of centering members , each having an upwardly projected pin , are oppositely arranged and adapted to move toward and apart with each other so as to abut on the longitudinal and transverse side edges of web on the table with said pin for centering . it is preferable and sufficient to move said centering members by means of a belt and pulleys for treating a light web .