In this study, we report that effects of a filler with a high coefficient of thermal expansion on a sealant for high-temperature ($750{sim}850^{circ}C$) SOFC. We designed a $SiO_2-BaO-ZnO-B_2O_3-Al_2O_3$ glass system with a softening temperature higher than $750^{circ}C$. The properties of the glass system show not only low volumetric shrinking but also low swelling. The glass system did not create a crystal phase during along-term heat treatment. We fabricated a seal gasket with 0, 10, 15, and 20 wt% cristobalite added as filler materials with glass powder. The coefficient of thermal expansion of the seal gasket increased according to cristobalite content. During along-term heat treatment, the leak rate decreased by about 5% after a heat treatment in an oxidizing atmosphere at $750^{circ}C$ for 2000 h, also decreasing by about 6% after a heat treatment in a reducing atmosphere at $750^{circ}C$ for 1000 h.