A catheter assembly (10) is provided with an inner cylinder (42) (protector) that covers at least a needle tip (17) of an inner needle (16) when the inner needle (16) is extracted. The inner cylinder (42) has a first region (74) and a second region (76), and an insertion hole (55) is formed between the first region (74) and the second region (76). At a portion in the axial direction of a wall section that forms the insertion hole (55), an interface section (110) between the first region (74) and the second region (76) is provided at a position offset towards the second region (76) with respect to the center of the insertion hole (55). This configuration enables suppression and prevention of the inner needle (16) being exposed from the protector even if the inner needle (16) reverts after the tip of the inner needle (16) is covered by the protector.
