To insert a sheet or the like between pages of text sheets while being continuously folded and stacked by a zigzag folding unit.

A method for bookbinding comprises the steps of separating the continuously printed text sheets 13 as pages of one book by a separating unit 40, folding predetermined pages of an original sheet size in a zigzag state to a flowing direction by a zigzag folding unit 50, operating an inserting unit 60 in the step of zigzag folding, inserting sheets 61 of another printing printed matter or the like between the pages of the text sheets, manufacturing gathered text sheets of one book, and then supplying the gathered sheets to a postprocessing unit by a conveyor 80.