In this paper, the writers described "Men-nasi Dansb" or planeless fault, discussing it's genesis, stratigraphical, geological and sedimentological features, as follows. 1) "Men-nasi Danso"is defined as, the displacement of strata or stratum in which the fault plane is completely, or at least tends to become obsolete. 2) The English translation for "Men-nasi Danso" is "Planeless Fault". 3) The Planeless fault may be due to fracturing after displacement during the process of sedimentation or soon after the sediments were deposited. As the result, the sediments of both walls, being still in a soft condition, should intermix, thus leading to the obliteration of the fault plane. 4) In the case of faults, in which the planes do not fuse with one another perfectly, a 1-2 mm thick clayey matrix may fill the interspace. Such a matrix may have soaken into the fault plane from above, squeezed out of both walls contemporaneously by the overlying weight, and thus result in the formation of a branching network which would aid in fusing the walls. This kind of a matrix should be distinguished from fault clay. 5) Planeless faults denote the following geological significances: (a) they are distributed in a definite horizon in some geological formations, (b) they occur in some areas where sudden disturbance in sedimentation took place, and, (c) there exists a mutual intimate relation between faulting and sedimentation. 6) The planeless fault may be distinguished from Shrock's (12) "penecontemporaneous fault", by the larger displacement (frequently attaining several meters) and by not exhibiting a fault plane.