Hollow spring utilizing seamless steel pipe

A hollow spring, comprising:

• a seamless steel pipe formed from a cylindrical steel material billet through a hot isostatic extrusion step, the seamless steel pipe having a hollow body in a shape of a coil or a bar or a bar with curved part, wherein
• a depth of a contiguous flaw formed on an inner periphery surface or an outer periphery surface of the steel pipe is 50 μm or less;
• the steel material is spring steel and the hollow body has compressive residual stress resulting from applying a surface treatment of either a quenching and tempering heat treatment plus nitriding treatment, or a carburization quenching and tempering heat treatment to the hollow body;
• and
• the hollow body in a shape of a coil or a bar or a bar with curved part has a cross-sectional outer contour in a shape of an ovoid in which a curvature radius on an inner diameter-side is smaller and a cross-sectional inner contour in a shape of a circle.