A wire material for a coil expander capable of manufacturing a coil expander that is unlikely to cause disadvantages such as abnormal wear and resultant reduction in strength The wire material for a coil expander is used for a coil expander of a combination piston ring having a piston ring and the coil expander for pressing and urging the piston ring outward in the radial direction thereof, wherein a cross-sectional shape thereof is a rectangular shape, and when the coil expander is formed by using the wire material for a coil expander, the surface of the wire material for a coil expander, constituting an outer peripheral surface of the coil expander, is formed into a convex curved shape.