In a medical guide wire, a core line securement portion (11) is provided in which the core wire (2) is fixed to an inner side of the helical coil body (3) so as to form a top leading end portion (10) between the core line securement (11) and the head plug (4). A flat plane structure (13) is formed in which the core wire (2) of the top leading end portion (10) gradually changes through stepped portions (S) so that the core wire (2) progressively reduces its thickness stepwisely while successively increasing its breadth stepwisely as approaching a foremost distal end of the top leading end portion (10). This provides an improved flexibility, sensitivity, torsibility and torque transmissibility with the flat plane structure (13) so as to be accessible to the diseased area with a high detective and ablative ability.