Provided is a method for increasing the thickness of the secondary cell wall of a plant. A method for increasing the thickness of the secondary cell wall of a plant by using a tracheary element differentiation (TED)-associated protein or a C-terminal fragment of the same, which comprises constructing a transgenic (Tg) plant containing, in an expressible manner, a DNA encoding protein(s) selected from the group consisting of a combination of TED6 with TED7, a C-terminal fragment of TED6, a C-terminal fragment of TED7, and a combination of a C-terminal fragment of TED6 with a C-terminal fragment of TED7, and expressing said DNA in said plant; the Tg plant; and a progeny, a cell, a tissue or a seed of the same.