We proposed and demonstrated a new modulation scheme of surface plasmon resonance (SPR) sensor for higher sensitivity. We fabricated novel SPR transducers with wedge membrane structure on the Au thin film (Otto configuration) for gas sensing application. The transducers are composed of SiO<sub>2</sub> / gas flowing layer / Au trilayers, fabricated by wafer bonding of SiO<sub>2</sub> substrate on Au thin film on Si substrate via the ball spacers to define the thickness (t) of the gas flowing layer. There is variation of t within the sample owing to the inhomogeneous bonding force during the adhesion process, leading to variation of t and modulating the SPR within the single transducer. The SPR was measured with attenuated total reflection with a glass prism. We measured SPR for different positions having different t by images detected by a camera with focusing and objective lenses. Asymmetric intensity distributions in the images were fitted with the simulated reflectivity, meaning that the obtained images correspond to the SPR near the cut-off condition in Otto configuration. We observed the shift of the intensity distribution upon the change of the excitation condition through different t. Based on the new modulation scheme, higher sensitivity is expected through analysis of the observed images.