A rocker arm that is rockably attached to a cylinder head actuates a valve by a guide groove that is formed at an end thereof engaging with an upper end of a valve stem, and a roller that is mounted in a roller housing portion of the arm main unit contacting against a cam and rocking. The guide groove is formed by press working ends of the arm main unit in the thickness direction of the plate material to form a pair of walls, and thereafter applying sandwiching pressure in the lateral direction to the walls to increase the height thereof.