A comment on the two modified numerical methods proposed by Yamada et al. is described. They proposed these methods under an understanding that Takata's central finite-difference method could not give a stable solution at higher Peclet numbers. But this understanding is questionable from the viewpoint of the author's published results, because they fixed a space increment for numerical calculation at 0.05 only and took no account of the significant effect of the space increment on the accuracy of numerical values. It is actually shown that Takata's method can give much more stable solutions than Yamada's two modified methods when the space increment is set at 0.01, even at a high Peclet number as high as 100.