Interfacial resistance is one of the severe problems in composite electrodes of all-solid-state batteries (ASSBs), especially oxide-type ASSBs. Conflicts between poor sinterability and possible unfavorable reaction with active materials limit applicable materials and processes. This work consists two parts: 1) distribution of ions and structural distortion around surface of solid electrolytes, 2) based on results of the first part, a novel approach is proposed to decrease grain boundary resistance among nonsintered solid electrolyte particles. The concept is successfully demonstrated, and the nonsintered grain boundary resistance of a highly conducting solid electrolyte (Li1.3Al0.3Ti1.7(PO4)3) was suppressed by being coated with poorly conducting solid electrolyte (Li2SiO3). The total conductivity (mainly contributed by grain boundary resistance) was improved by one order of magnitude. Increased total conductivity and variation of apparent activation energy are well explained from the viewpoint of defect chemistry.