All-solid-state batteries (ASSBs) have various problems associated with their usage that are normally not encountered in conventional lithium-ion batteries. Stress on interfaces between solid electrolytes and active materials is one of the key issues because the active materials change their volume during charging/discharging. In this work, first, we reveal that garnet-type solid electrolytes, Li6.5La3Zr1.5Ta0.5O12 (LLZT), prepared by the spark plasma sintering (SPS) technique, exhibit a residual tensile stress of more than 100 MPa in the direction of the SPS pressure. Then, the influence of the strain on ionic conduction is investigated in detail. It is demonstrated that the strain causes no change in the bulk resistance, while the grain boundary resistance increases in both the pre-exponential factor and the activation energy. The results suggest the importance of the strength of grain boundaries (including interfaces) for the practical application of ASSBs.