A paper tube manufacturing machine comprising: a transfer path (14 D ) for guiding transfer of a paper web, the paper web having multiple grooves which extend along a transfer direction of the paper web and increase the flexibility of the paper web; and a web bending device disposed on said transfer path (14 D ), for bending the paper web, wherein said web bending device includes: an endless garniture belt for transferring the paper web along said transfer path (14 D ); a tube forming section for forming the paper web continuously into a hollow cylindrical paper tube through the garniture belt while maintaining an inner surface of the paper web in a free state in the course of the transfer of the paper web, the paper tube having a lapped portion where both side edges of the paper web are lapped; an applicator for applying an adhesive to one of both side edges of the paper web before the lapped portion is formed, to thereby bond both side edges with each other in the lapped portion, characterized in further comprising: a guide path (14 U ) disposed upstream from the transfer path (14 D ), for guiding a web-like paper material to be formed as the paper web onto said transfer path (14 D ); and a groove forming device disposed on said guide path (14 U ), for forming the grooves in at least one of an inner surface and an outer surface of the paper material. a web bending device disposed on said transfer path (14 D ), for bending the paper web, wherein said web bending device includes: an endless garniture belt for transferring the paper web along said transfer path (14 D ); a tube forming section for forming the paper web continuously into a hollow cylindrical paper tube through the garniture belt while maintaining an inner surface of the paper web in a free state in the course of the transfer of the paper web, the paper tube having a lapped portion where both side edges of the paper web are lapped; an applicator for applying an adhesive to one of both side edges of the paper web before the lapped portion is formed, to thereby bond both side edges with each other in the lapped portion, characterized in further comprising: a guide path (14 U ) disposed upstream from the transfer path (14 D ), for guiding a web-like paper material to be formed as the paper web onto said transfer path (14 D ); and a groove forming device disposed on said guide path (14 U ), for forming the grooves in at least one of an inner surface and an outer surface of the paper material. an endless garniture belt for transferring the paper web along said transfer path (14 D ); a tube forming section for forming the paper web continuously into a hollow cylindrical paper tube through the garniture belt while maintaining an inner surface of the paper web in a free state in the course of the transfer of the paper web, the paper tube having a lapped portion where both side edges of the paper web are lapped; an applicator for applying an adhesive to one of both side edges of the paper web before the lapped portion is formed, to thereby bond both side edges with each other in the lapped portion, characterized in further comprising: a guide path (14 U ) disposed upstream from the transfer path (14 D ), for guiding a web-like paper material to be formed as the paper web onto said transfer path (14 D ); and a groove forming device disposed on said guide path (14 U ), for forming the grooves in at least one of an inner surface and an outer surface of the paper material. a guide path (14 U ) disposed upstream from the transfer path (14 D ), for guiding a web-like paper material to be formed as the paper web onto said transfer path (14 D ); and a groove forming device disposed on said guide path (14 U ), for forming the grooves in at least one of an inner surface and an outer surface of the paper material. <EOS>