Photoinduced expansion and contraction response in monolayers consisting of a poly(vinyl alcohol) derivative having an azobenzene (Az) side chain at the air-water interface was investigated in detail by surface potential measurements, Brewster angle microscopy (BAM), UV-visible absorption spectroscopy, and the macroscopic area estimations with a Langmuir film balance. Surface potential measurements confirmed the model of photostimulated motions previously supposed (Seki et al., Bull. Chem. Sec. Jpn. 1996, 69, 2375). The BAM observation combined with a movable minitrough allowed precise evaluations of the mechanical response of an isolated domain monolayer at zero pressure, which can be regarded as providing the inherent attributes. In comparison with the corresponding UV-visible spectroscopic data, the expansion process on UV light (365 nm) illumination was found to show a nonlinear response with photoreaction, which is characterized by the existence of an induction period. Initiation of the film expansion required ca. 40% conversion of the trans-to-cis photoisomerization. This method also confirmed a self-contracting motion on visible light (436 nm) illumination. In macroscopic area evaluations monitored at an applied surface pressure, the photoresponding behavior was found to include large artificial distortions. This work presents, for the first time, the inherent photomechanical response and its cooperativity in monolayers at the air-water interface.