A die for extrusion-molding in accordance with the present invention comprises: a molding groove part, a material supplying part, and an outer frame for fixing a die main body comprising the molding groove part and the material supplying part to the front end of an extrusion-molding apparatus, wherein the die main body has a form in which the plate-shaped molding groove part smaller than the material supplying part is provided at almost the middle of the plate-shaped material supplying part such that it protrudes from the material supplying part, the outer frame comprises: a die holding part which is provided such that it covers the material supplying part around the molding groove part; and a peripheral part which is provided near the die holding part, and supposing that the thickness of the molding groove part is X, the thickness of the material supplying part is Y, and the thickness of the die holding part of the outer frame is Z, these X, Y and Z are allowed to satisfy the inequalities (1) about 3≦X≦about 6, (2) about 5≦Y≦about 10, (3) about 3.5≦Z≦about 8.5, (4) about 0.8<Y/X<about 2.5, and (5) about 1<Z/X<about 2.