A spread monolayer of the amphiphilic block polymer poly(1,1-diethylsilabutane)-block-poly(2-hydroxyethyl methacrylate) (poly(SB-b-HEMA)) on the water surface was investigated by X-ray reflectivity (XR) measurements. Clear Kiessig fringes were observed up to the third order in the XR curve. This observation means the formation of the smooth-faced monolayer with high uniformity of thickness. The thickness of the monolayer increased with increasing surface pressure. Model filling with the two-layer model revealed that the upper layer is formed by the melt of the SE chains and that the lower layer consists of hydrated HEMA chains. The flexibility of the SE chain is thought to be an origin for the formation of the smooth, dense, and variable-thickness polymer monolayer on the water surface.