A catheter assembly (10) is provided with a protector (20) that when an inner needle (16) is extracted covers at least a needle tip (17) of the inner needle (16). The protector (20) has as inner cylinder (42) including an arm (46) that is releaseably connected to the base of a catheter hub (14) from the outside, and has an outer cylinder (62) inside of which the inner cylinder (42) is disposed and that is relatively displaceable in the axially line direction with respect to the inner cylinder (42). In the withdrawal operation for the inner needle (16), after the needle tip (17) of the inner needle (16) is stored in the protector (20), the outer cylinder (62) retracts with respect to the inner cylinder (42), and then as a result of the displacement of the arm (46) in the outward direction, the connection between the arm (46) and the catheter hub (14) is released.
