The buried-ridge structure was introduced to GaN-based laser diodes for the purpose of realizing an index-guided structure in order to control the difference of effective refractive indexes between the inside and outside of the lasing area and to stabilize the lasing mode. Low-temperature-grown AlxGa(1 - x)N was used as the burying layer, and it was found that the refractive-index difference can be controlled by adjusting the Al content in the AlxGa(1 - x)N burying layer independent of the ridge dimensions. As a result of fabricating LDs with different Al contents, we found that Al content should be more than 30% in order to form an index-guided structure. The optimum refractive-index difference was estimated by fitting with the calculation. The LD with a buried-ridge structure, in which Al in the burying layer was more than 30%, had desirable properties as an index-guided waveguide.