A method for producing a steel cord from 3 to 6 steel filaments on which a coating is applied to retain the adhesion to a rubber, the steel cord being destined for use to reinforce a rubber product having a flat section, comprising the following steps of preforming steel filaments at a ratio of more than 120% by twisting them together to form an open-structure steel cord; and passing the steel cord through a flattening apparatus provided at each of outlet and inlet thereof with a roller having a groove formed in the center of the circumference thereof, and between the inlet and outlet rollers with at least a roller of which the circumferential surface is flat, to repeatedly bend the steel cord vertically or horizontally.