[en] The Fukushima Dai-ichi Nuclear Power Station (FDNPS) accident in March 2011 led to releases of large amounts of artificial radionuclides including ¹³⁴Cs and ¹³⁷Cs (radiocaesium) into the environment. As of January 2015, based on TEPCO's monitoring data, ¹³⁷ activities in seawater had exponentially decreased more than 10 times compared to these before the FDNPS accident. However, the activities in sediments have decreased more slowly than those in seawater and they have had large fluctuations. It is necessary to consider the differences of grain size composition in sediments in order to compare the lateral and vertical distributions of radiocaesium in coastal sediments. This study was aimed at elucidating spatial variation and the behavior of radiocaesium activities and inventories for different grain sizes in sediments collected in the Fukushima coastal area in May 2014. The collected sediments were divided into four classes based on grain sizes using Several mesh sizes: granules (grain size larger than 2 mm); very coarse to coarse sand particles (1-2 mm); coarse to very fine sand particles (0.063-1 mm); and silt to clay particles (smaller than 0.063 mm). The percent values (%) for granules and very coarse sand, coarse to very fine sand, silt, and clay particles were: 0.0-23 and 0.0-39, 38-98, and 0.0-51, respectively. The ¹³⁷Cs activities (Bq kg^-1-dry) for granules, very coarse sand, coarse to very fine sand, and silt to clay particles were in the ranges: 7.1-8.4, 7.3-12, 8.5-609, and 43-1487, respectively. In the uppermost layer of sediments (0-3 cm), the ¹³⁷Cs inventories (Bq m^-2) for granules, very coarse sand, coarse to very fine sand, and silt to clay particles were 390, 460, 988-25779, 3998-11555, respectively. The inventories in a water depth of 60 m tended to be higher with finer particles. In a water depth of 120 m, the inventories with coarse to very fine particles were higher than those with slit to clay particles. It is suggested that coarse and fine particles adsorbed ¹³⁷Cs due to bottom current resuspension and lateral transport and then resedimentation onto the flat surface bottom. (author)