PURPOSE: To improve the light transmission efficiency of an internal light guide by limiting the position of a sealing material, which is provided between the internal light guide and an internal light guide sleeve and seals airtightly the internal light guide and the internal light guide sleeve, to a range near the incidence end face of the internal light guide.

CONSTITUTION: Generally, an angle θ of the light incident from an external light guide 14 to the normal of the incidence end face is limited to a certain value or smaller. The light made incident on an internal light guide 10 from the external light guide 14 does not strike a boundary surface 19 within a range of a width (b) from an incident end surface 18 of the internal light guide 10. Consequently, it is ideal that a solder material 170 is provided within a range of 0.4mm width from the incidence end surface 18 of the internal light guide 10 to seal airtightly the internal light guide 10 and an internal light guide sleeve 11. However, the solder material 170 is actually provided within a range of about 1mm width because it is difficult on production that the solder material 170 is provided within the range of 0.4mm width. In this case, the light transmission efficiency of the internal light guide 10 is improved considerably in comparison with the case where the solder material 170 is packed uniformly.