A rocker arm 6 that is rockably attached to a cylinder head 1 actuates a valve 4 by means of a guide groove 73b that is formed at an end thereof engaging with an upper end of a valve stem 41, and a roller 8 that is mounted in a roller housing portion 72 of the arm main unit 7 contacting against a cam 31 and rocking. The guide groove 73b is formed by press working ends of the arm main unit 7 in the thickness direction of the plate material to form a pair of walls 73d, and thereafter applying sandwiching pressure in the lateral direction to the walls 73d to increase the height thereof.