The increment of frictional deficiency by roughening the inner wall of spiral case is extracted from the optimum specific hydraulic energy deficiencies of a model Francis turbine that was measured by changing surface roughness of the spiral case. Assuming the free vortex flow in the spiral case, the frictional deficiency is predicted for the smooth and roughened spiral cases. By comparing the predicted deficiency with the measured increment of deficiency due to the roughness increase, the admissible roughness for the spiral case and the roughness conversion coefficient from the arithmetic mean roughness to the equivalent sand roughness are investigated.