To relieve stress concentration on a core part by provision of an aux. supporting part on a supporting metal piece, prevent slip-off of the core part and also exfoliation of the adhesion part, and increase the bending strength.

A polymer insulator tube 1 is configured with an FRP cylinder 2 and a sheath 3 made of rubber and composed of a sheath drum 4 and shade 5. Supporting metal pieces 6 are fixed the ends of the cylinder 2, wherein each metal piece 6 is composed of a body part 7 and an aux. supporting part 8 having a smaller wall thickness than the body part 7. The aux. supporting part 8 is arranged as protruding outward along the cylinder 2 from the inserting end 9 of the body 7, and its inside diameter is the same as the body 7. The heights of the body 7 and part 8 are decided so that h1/D and h2/D lie within the respective specified ranges, where D is the inside diameter of the metal piece 6, h1 is the height of the body 7, and h2 is the height of the part 8.