To understand unsteady flow phenomena around a body, it is important to measure time-resolved surface pressure distributions with high spatial resolution. Pressure Sensitive Paint (PSP) has a capability of providing high-resolution pressure images. In this study, we propose a new PSP technique to obtain time-resolved surface pressure distributions by using conditional sampling of time-series PSP images. To obtain high SNR data, the high-speed camera images are phase-averaged after image acquisition, using a simultaneously-measured pressure transducer data to detect a phase of each image. This method allows us to obtain high SNR images at arbitrary phase of the periodic phenomena using a simplify measurement system and image processing. The effectiveness of the present method was evaluated by applying it to a 3-D square cylinder in low-speed wind tunnel testing. As a result of this experiment, we could obtain time-resolved unsteady pressure images around the square cylinder caused by periodic vortex shedding.