When an insulator is used in a humid atmosphere, the cooling thereof causes a humidity condensation, which is deposited on the insulator surface. Drops of water formed by the humidity condensation are connected to each other that a water film substantially reducing the insulation resistance of the insulator is formed. In order to solve said problem, the inventive insulator is provided, at least partially, with a hydrophobe surface. In such a way, the water layers formed by condensation are drained off the surface or detached there from without connection and the drops of water produced by condensation are unable to be bound to each other, thereby making it possible to maintain a desired high insulation resistance.