The critical brush density, where transition from carpet-only structure to carpet/brush double-layered structure occurs, was estimated for a weakly ionic amphiphilic diblock copolymer, (diethylsilacyclobutane)(34)-b-(methacrylic acid)(50), monolayer on water by an in situ X-ray reflectivity technique. The brush density in the monolayer was controlled from 0.11 to 0.60 brush chain/nm(2) by changing surface pressure and mixing a poly(diethylsilacyclobutane) homopolymer separately synthesized. Only a carpet layer was formed at a low brush density condition, but a carpet and brush double layer was found for a higher brush density of more than 0.48/nm(2). This brush density, which is fairly high, would be valuable for discussing the polyelectrolyte brush nanostructure.