To solve the problems of the Mellor-Yamada-Nakanishi-Niino (MYNN) model, we attempted to modify it by following the method proposed by Canuto et al. (2008) for the stable stratification case. In contrast to the original MYNN model, the modified model has no critical Richardson number, and the effect of turbulent motions remains for all Richardson number values in the level 2 model. Furthermore, the modified model possesses an advantage even for the level 2.5 and level 3 models. The flux Richardson number is unbounded, and a lower limit of the turbulent kinetic energy must be imposed in the original MYNN model; these features do not appear in the modified model.