A spiral cap includes: a cap body; a coupling portion provided on the cap body and coupled to an end portion of a tube body which is rotatably attached to an insertion section of an introducing device; a fin connector to which an end portion of a main fin portion is connected, the main fin portion being provided on an outer surface of the tube body in a spiral shape about a longitudinal axis of the tube body; and an insertion assisting fin portion provided on an outer surface of the cap body and forming a fin in cooperation with the main fin portion connected to the fin connector.