A medical device and a treatment method are disclosed that can be applied to body lumina with a wide range of inside diameters and can enhance a property for suction of a substance from the inside of a body lumen by restricting flow within the body lumen. The medical device includes an elongated shaft section； an expanding section that is an elastically deformable tubular body provided with a plurality of openings； a flexibly deformable tubular cover section that is coupled to an end portion on a proximal side of the expanding section, surrounds an outer periphery of the expanding section on the proximal side, but does not surround, and externally exposes, an outer periphery of the expanding section on a distal side； and a sheath capable of accommodating the expanding section and the cover section in a diameter-reduced state.
