A damper includes a hollow outer elongated body; a hollow inner elongated body; a liquid viscous body or liquid viscoelastic body disposed in a gap between an inner surface of the elongated body and an outer surface of the elongated body in such a manner as to be in contact with the inner surface and the outer surface of these elongated bodies, respectively; and first and second attaching structures. Further, a controller may be provided to control an increase or decrease in the internal pressure of the liquid viscous body or the liquid viscoelastic body.