A front portion of a tubular inner needle proximal end portion disposed at a proximal end of the inner needle is attached to a rear end portion of a tubular adaptor, a tubular outer needle proximal end portion disposed at a proximal end of the outer needle is inserted from a distal end of the adaptor, and a distal end of the inner needle is caused to protrude from a distal end of the outer needle. A restricting plate having an insertion hole through which the outer needle proximal end portion is inserted is inserted into the adaptor from an opening formed in an upper surface of the adaptor. A protrusion formed on a lower surface of the insertion hole protrudes upward and engages with one of the grooves of the outer needle proximal end portion. When the restricting plate is pushed down, the groove of the outer needle proximal end portion and the protrusion of the restricting plate are disengaged from each other and the inner needle is set to the unlocked state in which the inner needle is able to be advanced or retracted relative to the outer needle.