Abstract: The needle protection cover of a catheter assembly is provided with: an inner cylinder; an outer cylinder capable of being axially displaced relative to the inner cylinder; a blocking body disposed in the inner cylinder in a displaceable manner; and a lock mechanism for restraining the blocking body at the position where the blocking body blocks an inner needle passage. As an inner needle is pulled out, the blocking body is pressed by contact sections formed on the outer cylinder, thereby being moved from an initial position to a blocking position, and the blocking body is held at the blocking position by the lock mechanism.