In the 9th exploration on April 21, 2017 we introduced a new detector in order to increase the detection efficiency and to improve the statistical accuracy. We have chosen a commercially available Hamamatsu C1237-10 which consists of a large cylindrical CsI(Tl) scintillator of a size of φ110 mm×25 mm with MPPC (Multi-Pixel Photon Counter). As a result, we could reduce the measurement time by half while we could improve the statistical accuracy twice. On May 21, 2018, July 14, 2018, December 22, 2018, and July 13, 2019 we went out to survey the reefs lying between Fukushima Daiichi Nuclear Power Plant (FD1NPP) and Fukushima Daini Nuclear Power Plant (FD2NPP). Figure 1 shows the location of the measured points in these five explorations. It is found that the amount of the residual 137Cs is in the order of 104 Bq/m2 and the spatial distribution of residual 137Cs is uneven. These two facts are consistent with what we have found in our earlier explorations using our previous detector.