Namely, an example of such structure is shown in the attached FIG. 3 wherein, when inserting an optical fiber 23 in a penetration hole 22 penetrating through the central axis portion of an insulator 2, the optical fiber 23 having a coating portion 23-1 is fixed at the middle portion of the penetration hole 22 by filling a rubbery elastomer or a resin, such as silicone rubber or epoxy resin, and a