The effect of SiO2 impurity on the grain-boundary conduction in Sr- and Mg-doped lanthanum gallate (La0.8Sr0.2Ga0.8Mg0.2O3-delta, LSGM) was studied. The grain-boundary conduction of LSGM was slightly affected, even by the addition of 500-2000 ppm SiO2, while the apparent grain-boundary resistivity was increased up to similar to 780 times in gadolinia-doped ceria by the doping of 500-2000 ppm SiO2. The high tolerance of the grain-boundary conduction against siliceous impurity in the LSGM specimen was explained by the gathering of the acidic siliceous phase near the basic SrO component.