In a medical guide wire 1, a flexible core wire 2 is made of austenitic stainless steel wire treated with a solid solution procedure, and drawn with a whole cross sectional reduction ratio as 90%-97.6%. Each time when the core wire 2 is subjected to a series of mechanical procedures, the core wire 2 is heat treated repetitively at low temperature subsequent to the mechanical procedures to improve a tensile strength characteristic of the core wire 2. The core wire 2 is twisted under the low heat treatment. Effectively used is a heat generated from a synthetic layer 6 when coating the layer 6 on the helical spring body 3. This makes it possible to increase the characteristic of the tensile strength of the core wire 2 with its thermal conductivity taken into consideration. Upon improving the tensile strength of the core wire 2, optimal conditions are achieved by observing a relationship between the tensile strength and the temperature which the core wire 2 exhibits when tightly drawn as a wrought-out procedure.