In the 12th report, we proposed a theoretical solution for the worst stress when a uniformly distributed load acts upon thin rim gears which have center web. Furthermore we examined its validity by three-dimensional photo-elastic experiment. We also pointed out the importance of finding load distribution on tooth surface. In this report, we calculated this stress distribution on tooth surface and synthesize the results to obtain a theoretical solution for the worst stress on the root fillet of spur gears which have a thin rim and a center web. We also found out characteristics of distribution of load on tooth surface, amplification of root fillet stress for each combination of b/a, h/b, h/m. We finally estimated the optimum relation of rim/module.