Provided are a guide wire having high workability and whereby erroneous puncture by a needle part for puncturing is suppressed, and safety can be ensured, a medical device, and a treatment method. A guide wire (10) for guiding a dilator (40) inserted in a living body, the guide wire (10) having a shaft part (11), a puncture part (15) fixed to a distal part of the shaft part (11) and provided with a needle part (16) on a distal side thereof for forming a hole in a biological tissue, and a deformable deformation part (30) fixed to the shaft part (11) or the puncture part (15), the deformation part (30) positioned inside the dilator (40) limiting flexing of the distal part of the shaft part (11).
