A thin and long groove (35) is formed so as to link an Ag layer (32) and an exposed portion of a circuit layer surrounding the Ag layer (32). The groove (35) is a thin and long recess extending from the surface of the Ag layer (32) through a glass layer (31) and an aluminum oxide coating (12A) to the surface (12a) of the circuit layer (12). An extended part (36), in which a part of the Ag layer (32) is extended along the inner surface (35a) of the groove (35), is formed in the groove (35). The extended part (36) establishes a direct electrical connection between the Ag layer (32) and the circuit layer (12) at the portion at which the groove (35) is formed, the connection being established using Ag, which has low electrical resistance.