We have investigated the local free volume and its thermal fluctuation in an amorphous polymethylsilsesquioxane (PMSQ) consisting only of T-3 (CH3SiO3/2) and T-2 (CH3Si(OH)O-2/2) units using photoisomerization of azobenzene molecularly dispersed in the PMSQ films as a photoprobe over a wide temperature range (4-300 K). Photoisomerization profiles are discussed an the basis of three parameters, i.e., first-order plots, final cis fractions, and three-component analysis, for uncured and silanol-condensation-cured PMSQ＇s. The final cis fraction in the uncured PMSQ decreases markedly below 250 K, which has never been observed in linear polysiloxanes, MQ resins, or carbon-based polymers. The effect of low-molecular-weight components in the PMSQ was evaluated using fractionated PMSQ＇s by molecular weight. The effect of hydrogen bonding by a silanol group was also studied by temperature dependence of infrared spectra. These results suggest that the low-molecular-weight components with silanol groups fill local vacant space resulting in the anomalous decrease in the final cia fraction for the uncured PMSQ below 250 K.