Coil spring

A coil spring comprising a wire rod with a first end and a second end and including a first end turn part including the first end of the wire rod, a second end turn part including the second end of the wire rod, and an effective spring part including a plurality of coil portions formed between the first end turn part and the second end turn part, with respective gaps between coil portions adjacent to each other,

• the wire rod comprising:
• a round section portion including a round first cross section perpendicular to a longitudinal direction of the wire rod;
• a quadrangle section portion including a quadrangle second section perpendicular to the longitudinal direction, and including at least a first coil portion and a second coil portion, a width and a thickness of the second section each being less than a diameter of the first section, and an area of the second section is less than an area of a square inscribed in a circle of the diameter of the first section;
• and
• a variable section portion formed between the round section portion and the quadrangle section portion of the wire rod, a cross section thereof perpendicular to the longitudinal direction varies from circular to quadrangle and an area of the cross-section decreases, from the round section portion to the quadrangle section portion.