In order to decommission the Fukushima Daiichi Nuclear Power Plant, we have developed a method to estimate the radiation source distribution accurately by searching the map in a limited time. In the proposed method, multiple robots estimate the radiation source distribution by the coarse-to-fine search. The coarse-to-fine search is to estimate the distribution by coarse search of the map, and then to search finely the high radiation source area based on the estimation. As a result of examining the efficiency of the search by multiple robots, the search time was only 1.5% compared to the case that the map was completely divided equally. As a result of estimating the distribution of radiation source with and without the coarse-to-fine search, the coarse-to-fine search is more accurate.