Like natural seismicity, induced seismicity caused by injection also shows a power law size distribution, and its gradient b-value is used for seismic hazard analysis. Despite the well-known result that b-value is negatively correlated with differential stress for natural earthquakes, there is no similar correlation for b-value variations because the differential stress is nearly constant for injection-induced seismicity, we thus investigate the b-value dependence on the relative shear stress acting on existing fractures, using the fault orientation and in situ stress information. The seismicity occurring along existing fractures having high shear stress has significantly lower b-values than does that associated with lower shear stress fractures. We examine the b-value dependency on slightly changing differential stress, but the relationship is not clear. The b-value for injection-induced seismicity is dependent on relative shear stress on faults, which is a novel physical explanation for the b-value variations of induced seismicity.