<p>　Recently, with the progress of research on earthquake ground motion, it has become necessary to increase the out-of-plane shear force used in the design of reinforced concrete flat plates such as foundation slabs and underground outer walls. There is a concern that out-of-plane shear failure occurs in these flat plates of existing buildings with insufficient amount of shear reinforcement during coming severe earthquake. However, effective seismic retrofitting of foundation slabs or underground outer walls is difficult because the construction work can be completed only from the inside of building. To address this problem, a post-installed shear reinforcement (PSR) method was developed. </p><p>　The PSR consists of a deformed bar and ceramic or steel anchorages, called “ceramic cap bar (CCb)” or “steel cap bar (SCb)”, respectively, at both ends of the bar. In this method, after drilling holes in the existing foundation slabs or the underground outer walls, PSR is inserted into the hole from the inner space of the building, and finally grout is injected to integrate the PSR with the structure. </p><p>　In order to grasp the out-of-plane shear characteristics of a flat plate reinforced with PSR and to develop a formula to predict its shear strength, 31 test specimens were experimented. The main parameters were the presence or absence of PSR, the material of the anchorage, the length and amount of PSR, the shear span ratio and the size of the specimen.</p><p>　The major findings from this study are as follows: </p><p>1)　In the member with PSR, shear cracks progressed and widened in the hinge region where the area around the tip of PSR is near the bending tension side. The cracks then developed in the horizontal direction so as to connect the PSR tip and reached the maximum strength. After the maximum strength, these horizontal cracks developed and widened, and the damage progressed.</p><p>2)　Based on Arakawa’s formula, a formula to predict the out-of-plane shear strength of the flat plate seismically retrofitted with PSR was proposed. This formula includes the “shear reinforcement effectiveness ratio”, which accounts for strength development characteristics of PSR, and the “effective section ratio”, which indicates that the outer concrete from the tip of PSR does not contribute to the shear resistance. With this formula, the out-of-plane shear strength was predicted on the safe side.</p>