To improve efficiency and service life of a fuel cell, it is important to understand the dynamic behavior of oxygen molecules in the cell. In other words, we need to know the oxygen concentration distribution over electrode planes and inside diffused layers in addition to its time variation. We applied the Pressure-Sensitive Paint (PSP) technique to this problem, attempting to measure distribution of partial pressure oxygen along micro channels of a fuel cell. To prevent the effect of water produced by the power generation process, we developed a water-repellent PSP that can withstand heat and humidity produced by power generation. Using this paint, partial oxygen distribution along the micro channels was successfully visualized, although there were noticeable effects of local temperature distribution on the absolute accuracy for the conditions with higher current densities.