PURPOSE: To wrinkle fiber sheet so as to impart variable density patterns having natural depth by fluctuating the fiber sheet as it is or after pretreatment between kneading rolls to subject it to structural change through wrinkling it and loosening fiber bundle at the same time and then by coloring it.

CONSTITUTION: The method of this invention is to subject fiber sheet P such as paper as it is or after pretreatment such as heating, cooling, moistening or the like to structural change gripping the sheet P with a pair of kneading rolls 2 and 3 both having a corrugated surface installed in a kneading machine F and wrinkling it in a wrenched state by fluctuating it in a cross direction to its running one while adjusting the nipping force of the kneading rolls so as to form natural wrinkles in both warps and wefts and loosening fiber bundle by relaxing entanglement and association of fibers at the same time, followed by coloring the loosened part of fibers e.g. by coating it with a coloring liquor including dye, pigment or the like so as to have deeper color than that in a part not subjected to the kneading treatment to impart variable color density wrinkles having natural depth to the fiber sheet. The processed fiber sheet is suitable for packaging paper, pouches, tapestry or the like.