The 1977-1978 activity of Usu Volcano was characterized by a series of major plinian eruptions in August 1977 and a number of phreatic and phreatomagmatic eruptions from November 1977 to October 1978. Pyroclastic deposits of this activity are well preserved and exposed near the craters. The successive pyroclastic deposits near the craters are subdivided into four members based on their distributions and lithofacies. They are member 1, member 2, member 3 and member 4 indescending order. Member 1, the uppermost one of the members, is dominated by pyroclastic surge deposits made up of coarse ash. Member 2 is characterized by some major beds made up of mixture of ballistic bomb, pyroclastic fall deposits and pyroclastic surge deposits. Member 3 is dominated by pyroclastic surge deposits composed mainly of fine ash. The lowest part of Member 3 contains small accretionary lapilli and vesiculated tuffs indicating that the eruptions occured in a wet area. Member 4, the lowest one of the members, consists of pumice-rich pyroclastic fall deposits and minor surge deposits. The eruption dates and vents of some major beds were identified by comparing the field observational data and records published just after the eruptions. From the chronological data of the major beds, the source craters and the eruptive term of each member are ditermined as follows : member 1, Crater N and between 17 and 27 in October 1978 ; member 2, Craters K-M' and between 16 August and 17 Octber in 1978 ; member 3, Craters J-L and between 5 July and 16 August in 1978 ; member 4, Craters 1, 2 and 3 in August 1977. The total volume of deposits ejected from Craters A-N, mainly from Craters J-M′ (Ginnuma Crater), is estimated at 2.1 × 10⁷ m³. A quarter of that deposits in the summit crater of Usu Volcano.