A steel for spring includes C: 0.5 to 0.6%, Si: 1.0 to 1.8%, Mn: 0.1 to 1.0%, Cr: 0.1 to 1.0%, P: 0.035% or less, S: 0.035% or less, by mass %, and a balance of iron and inevitable impurities, as the overall composition, wherein an area ratio of an internal structure on an optional cross section comprises bainite: 65% or more, retained austenite: 6 to 13%, and a balance of martensite, and average C content in the retained austenite is 0.65 to 1.7%. The steel for spring can have high strength in which tensile strength is 1800 MPa or more and have high ductility.