To provide a core material for manufacturing a catheter tube, to be easily stretched with low loads, to be hardly stuck to a resin, to be easily made straight, to be hardly injured, to be easily and inexpensively manufactured, suitable for manufacturing the catheter tube by forming a resin coating layer on the outer surface of the linear core material and pulling out the core material thereafter.

The base metal of the core material is soft iron including carbon less than 0.03 wt.%. A Zn-plated layer with 1-5 m thickness is formed on the surface of the core material, and a diffusion layer in which Zn is diffused in Fe is formed on the inner side of the Zn-plated layer of the core material. Accordingly, a wire made of 0.02C electromagnetic soft iron is softened and annealed in an inert gas at 700 C or higher (a), extended to make the wire with 0.4-2 mm diameter (b), and the surface of the wire is electrolytically plated with Zn to form the Zn-plated layer with 1-5 m thickness (c). Then, the wire is diffusion-annealed in the inert gas at 420-600 C.