Ionic amphiphilic diblock copolymer poly(hydrogenated isoprene)-b-poly(styrenesulfonate) was synthesized by living anionic polymerization. The nanostructure of its monolayer on the water surface, especially that of the poly(styrenesulfonate) brush under the water surface, was investigated using X-ray reflectivity as a function of the added salt concentration with different kinds of salts (NaCl, KCl, LiCl, CaCl2, and MgCl2). The critical salt concentration (csc) at which the polyelectrolyte brush starts to shrink by salt effect was found for many kinds of salts. The csc values were found to be in the order of Li+ < Na+ < K+ for monovalent ions and Mg2+ < Ca2+ for divalent ions. This order is in agreement with the order proposed in the well-known Hofmeister series, and this means that the so-called structure-maker ions, such as Li+, can go into the inside of the brush layer more easily than structure-breaker ions like K+. Although no direct evidence has been found, it is reasonable to think that the water structure inside the brush is something special because the analytical ion concentration in the brush layer is very high (ca. 2 M).