X-ray reflectivity (XR) measurements were carried out for monolayers of poly(vinyl alcohol) containing a photochromic azobenzene side chain (6Az10-PVA) on a water surface. Kiessig fringes were observed for specular measurement for 6Az10-PVA monolayers on a water surface. By the irradiation of visible and ultraviolet light, azobenzene shows a photoisomeric conformational change between trans and cis forms, respectively. The structural change of a 6Az10-PVA monolayer on a water surface was detected by in-situ XR measurement. By analyzing the XR data, it was indicated that the thickness of the monolayer became thicker for the trans form than for the cis form. This reflects that the side chain should stretch to the direction perpendicular to the water surface with the conformational change from cis to trans form, in addition to the longer chain length for the trans form.