We perform quantum chemical calculations for the Cs+, K+, and Na+ complexes of norbadione A (NBA), a pigment molecule in mushrooms known to accumulate Cs+. A numerical two-step approach, by Ota et al., is employed to examine its alkali-metal-cation complexation selectivity in aqueous solutions. Applying it to the neutral, di- and tetra-deprotonated NBAs, we confirm that the complexation selectivity on Cs+ emerges only in high pHs, in which the di-protonated NBA dominates, in agreement with experimental results. This is the first demonstration of the approach for a biological molecule whose selectivity is known to be anomalous.