A medical instrument introduction device, includes an outer tube portion that is within a body wall； and an inner needle portion that is inserted through the outer tube portion, wherein the outer tube portion includes a base portion disposed at a proximal side of the outer tube portion； and a tubular conduit portion, the conduit portion being formed of a stretchable material, a proximal end portion of the conduit portion being connected to the base portion, wherein the conduit portion has a bellows folded shape in a natural state, a dimension of the conduit portion in an axial direction being capable of being increased as the folded shape is stretched by the inserted inner needle portion, and a force needed for stretching the folded shape being gradually decreased in a direction from the proximal end portion to a distal end portion of the conduit portion.
