PURPOSE: To produce imitation wood coated with flat grain pattern or concentric circular pattern akin to natural wood by moving laterally or vertically the uneven die part provided in a middle die for making up the wood grain pattern when synthetic resin is extruded from an extrusion molding machine.

CONSTITUTION: In a continuous extrusion molding machine, a middle die 2 is provided to form the necessary surface of the material 4 in a wavy or partially concentric circular shape at the neighborhood of a main die 1 for expanding the material 4. This die 2 is a movable section 5 capable of moving a little laterally or vertically. When the movable section 5 having a wavy pattern (a) or concentric circular pattern (b) section is moved a little laterally or vertically, the uneven streak pattern created describes a zigzag or concentric circular pattern to obtain wood approximate to the actual grain patten. When the synthetic resin is fed into a cooling die 3 finally, it is compresed to the desired finish shape, cooled and cured into a completed article.