A treatment method and medical device are disclosed for cutting a substance inside a body lumen. The medical device for cutting substances inside a body lumen including a rotatable tubular drive shaft； an expanding part connected to a distal side of the drive shaft； a cutting part covering the expanding part； and an elongated tube extending through the drive shaft and connected to the expanding part, wherein a distal end of the expanding part is placed distal to a distal end of the cutting part.
