This paper discusses the slope development in a small basin formed since the Last Glacial Stage in the non-glaciated eastern part of the Tokachi Plain, Hokkaido. To minimize the effect on slope development of factors such as change in base-level and debris supply, valley-side slopes of a small tributary basin developed in the fluvial terrace of the pre-Last Interglacial age (ca. 150, 000y. B. P.) and accompanied by an alluvial cone which had been formed on the terrace surface of the mid-Last Glacial age (ca. 40, 000y. B. P.) have been selected for the present analysis.<br> Valley-side slopes in the basin studied are roughly classified into two slope units, i.e. upper sideslope and lower sideslope, by the clear break of slope gradients. Upper sideslopes are further divided into slope units by the indistinct slope breaks.<br> Stratigraphically, each slope unit has its own airborne tephra cover. Uppermost “upper sideslopes (Ia)” are covered by the Shikotsu pumice fall “1” (Spfa 1, ca. 32, 000y. B. P.). Middle or lowermost “upper sideslope (Ib, c)” are covered by the Eniwa “a” pumice fall (En-a, ca. 15, 000y. B. P.) and the Tarumae “d” pumice fall (Ta-d, ca. 9, 000y. B. P.). In contrast, almost all of the “lower sideslopes” were covered by tephra layers after the Tokachi “c-2” ash fall (To-c2, ca. 5, 000y. B. P.), and no tephra layers cover recently failed slopes. This tephrochronological analysis has revealed an upper part of the valley-side slopes older than the lower part. That is, upper sideslopes had been formed in the time ranging the Last Glacial Stage to the Postglacial, and lower sideslopes in the Postglacial. These facts demonstrate that tephrochronological analysis is obviously applicable to the study of slope development as well as to that of terrace chronology.<br> The slope development of the basin studied in and after the Last Glacial Stage is summarized as follows. During the Last Glacial Stage, upperward erosion of valley-heads and following degradation of channelways provoked slope failures and surface denudation, which resulted in slope retreat and the formation of upper sideslopes. On the other hand, in the Postglacial, the bottomland was widened by lateral erosion of meandering channelways in a, graded state ; slope failures occurred as a result of wash-out of footslopes, and thus lower sideslopes originated. Every slope unit of the basin was completed by the mid-Holocene (ca. 5, 000y. B. P.). Because the effects of the base level change and debris supply change are neglected in the study basin, the difference in slope development processes in the two stages is attributable to changes in channel conditions due to climatic changes such as an increase in precipitation. It is therefore concluded that the valley-side slopes were formed by the changes in the magnitude and the mode of denudational processes, reflecting climatic change during and/or after the Last Glacial Stage.