A catheter includes: an outer tube； an inner tube provided inside of the outer tube； and a branched part that separates the outer tube and the inner tube from each other on a first end side thereof, wherein a ratio between cross-sectional areas of a first lumen formed at least by inside of the outer tube and outside of the inner tube and a second lumen formed within the inner tube is set such that a ratio between cross-sectional areas of the outer tube and the inner tube in the branched part is equal to or greater than the ratio between the cross-sectional areas of the first lumen and the second lumen in a state where the inner tube is arranged inside of the outer tube in a length direction in which the outer tube and the inner tube extend.
