A dilator, dilating a narrowed part of a paranasal sinus, includes an end effector, and the end effector includes a pair of treatment pieces openable and closable relative to each other. Each of the treatment pieces includes a treatment surface facing a side on which the treatment piece opens, and each of the treatment surfaces includes an inclined surface inclined to be closer to a side on which the treatment piece closes as a distance to a distal end of the treatment piece decreases. The pair of inclined surfaces become parallel or substantially parallel to each other when the treatment pieces are opened to a maximum angle. The end effector opens anisotropically.
