The Middle Miocene Katsumoto Formation, distributed in the northern and central part of Iki Island, Nagasaki Prefecture, has been considered as a typical back-arc basin sequence. This study aims to describe sedimentary facies, ichinofacies and paleocurrent system and to discuss sedimentary environments and depositional systems. The Katsumoto Formation, attaining more than 450m in thickness, shows a coarsening- and thickening-upward sequence as a whole. It begins with mud-dominated facies of interbedded sandstone and mudstone, passes upward into medium-bedded, medium-grained sandstone and siltstone facies, and ends with thick-bedded, medium- to coarse-grained sandstone facies at top. Six sedimentary facies recognized. Their lithologies are summarized as follows; Facies A: mainly well-sorted, medium to very coarse sandstone with planer to tabular cross-stratification, Facies B: well-sorted, very fine to medium sandstone with amalgamated hummocky cross-stratification (HCS), Facies Ca: interbedded HCS sandstone and siltstone, Facies Cb: thinly interbedded HCS sandstone and siltstone, Facies D: mottled sandy siltstone with thin very fine-grained sandstone, Facies Fa: slump breccia, Facies Fb: slump folded thinly interbedded sandstone and mudstone, and Facies F: interbedded very fine-grained turbidite sandstone and mudstone. The sedimentary environments inferred from Facies A to F are summarized as follows; Facies A: upper shoreface, Facies B: lower shoreface, Facies Ca and Cb: inner shelf, Facies D: outer shelf, Facies Ea: shelf margin, Facies Eb: continental slope, and Facies F: basin plain. Fossil evidence, such as burrowing organisms and benthonic foraminiferas obtained from the formation, seems to coincide with these environments. Eastward and northeastward paleoflows are dominant in the lower, and the middle and upper parts of the formation, respectively. It can be concluded, therefore, that the Katsumoto Formation represents a facies succession from basin plain to tide-dominated shallow marine sediments due to northeastward progradation process. The northeastward progradation system is also recognized in the regional area of the Tsushima Basin in the Middle Miocene time.