Abstract: Apparatus for manufacturing a trapezoidally-shaped paper bag comprises a cutting and processing device to form a continuous line of blanks in a paper strip, left and right guide rail units for raising up the side flaps of successive blanks, left and right folding ruler members arranged in an inverted V in the direction of advance of the paper strip, a base rod for causing forward and backward reciprocating movement of the left and right ruler members on the upper surface of the blank being processed between the guide rail units, and inwardly and outwardly reciprocating left and right sliding plates for folding up the side folding flaps of the blank by moving inwardly over the left and right ruler members, respectively, in an overlapping relation.