Significant changes in the thickness of poly(N-isopropylacrylamide) solid ultrathin films have been observed by X-ray reflectivity as a result of changing the temperature between 15 and 60 degrees C in an uncontrolled atmosphere. As the property of the film switches between hydrophobic and hydrophilic nature at around 33 degrees C, the present letter reports that an ultrathin film of around 200 angstrom exhibits nearly a 7% change in thickness even in an ordinary atmosphere and that cooling and heating exhibit different dependence; i.e., one can confirm the switching point in the cooling run, but the dependence is linear in the heating run. The phenomena are reproducible and repeatable.