For the hybrid structures of steel and concrete, the connection system of steel and concrete is very important, to ensure its performance in terms of stiffness and static strength. It is well known empirically that the shear strength of the connectors may extremely increase when they are constrained by the steel such as reinforced bars, but this phenomenon is not sufficiently and quantitively clarified. Thus herein, the fundamental behavior of shear connectors of steel and concrete under constrains is clarified. In this experiment the relationship between constraint and shear resistance is assessed in the monotonic pull test and the increase of the strength is quantitively clarifed.