PURPOSE: To prevent the generation of air bubbles produced at glass bonding, by forming a plurality of blcks formsing a front core with a long-sized member, forming a vapor-deposited film at an end surface through the formation of a bridge to the one block and cutting the member while removing the bridge after the glass bonding.

CONSTITUTION: While a groove is interposed at both sides of a member A10, a bridge 18 in which the tip has the same height as a gap forming section 15 is formed and an exclusive mask is used for the bridge 18 to form a vapor- deposited film 16. The vapor-deposited film 16 positions the members A10 and B11 for glass bonding and since the grooves 17 exist even if a gas is generated from the film 16, no air bubble is caused in blocks A6 and B7 to be a front core 4. Thus, no defect due to the air bubbles is produced. Even if Al2O3 with much gas generation is used, the front core 4 without air bubbles can be obtained with the means as mentioned above.