Clupak paper is manufactured using a gap-former paper-making machine equipped with a Clupak system, in such a way that its longitudinal tensile energy absorption index and lateral tensile energy absorption index as specified in JIS P8113 become 2.5 J/g or more and 1.0 J/g or more, respectively. The paper has excellent strength characteristics in both the longitudinal and lateral directions, especially in the longitudinal direction so that when the Clupak paper can be processed into a sack, the sack rarely breaks.