PROBLEM TO BE SOLVED: To provide a method and a device for producing an extrusion molded product by which hard synthetic resin can be flowed smoothly, and an extrusion molded product having smooth surface can be obtained. SOLUTION: The extrusion molding device 1 comprises a die 2 and a sizing 3 disposed at the tip end of an extruder 5. The sizing 3 has a shaping flow passage part 32 and an enlargement flow passage part 31 enlarged toward the extruding direction. The inner wall faces 311, 321 of the enlargement flow passage part 31 and the shaping flow passage part 32 are formed smooth. The hard synthetic resin 41 which is heated and melt is extruded from an orifice 212, cooled and solidified by the sizing 3 to form a smooth surface 411 by transferring the inner wall face 311 at the enlargement flow passage part 31 to obtain the extrusion molded product 4 by shaping the smooth surface 411 in the shaping flow passage part 32.