Method for manufacturing long member

A method for manufacturing a long member made of a polymer material through extrusion molding, the long member including a holding portion including:

• a bottom wall;
• a first lateral wall and a second lateral wall which stand from both ends of the bottom wall;
• and a metal core member that is embedded in the bottom wall, the first lateral wall, and the second lateral wall from the bottom wall to the first lateral wall and the second lateral wall, an angle between the bottom wall and the first lateral wall being constant throughout in a longitudinal direction, and an angle between the bottom wall and the second lateral wall being partially varied in the longitudinal direction, the method comprising:
• a first step of pre-forming the angle between the bottom wall and the first lateral wall and the angle between the bottom wall and the second lateral wall to be constant in the longitudinal direction and equal to or larger than final maximum angles thereof;
• a second step of regulating a boundary between the bottom wall and the second lateral wall from moving toward the first lateral wall after the first step;
• and
• a third step of forming the angle between the bottom wall and the second lateral wall into a final angle by partially varying, in the longitudinal direction, the angle between the bottom wall and the second lateral wall.