Sub-Plinian eruptions often repeat with short intervals and the magma discharge rate during the eruptions changes. The mechanism that controls the variation in the discharge rate remains unclear, but its understanding is essential for predicting the evolution of explosive eruptions and mitigating the disaster. Here, we petrologically investigated the magma ascent processes during the 1977 eruption of Usu volcano, with four sub-Plinian and two relatively small pumice eruptions. The water contents of the melt inclusions in the first sub-Plinian eruption indicate a pressure of 100-125 MPa with the assumption of water saturation. From the second to the fourth sub-Plinian eruptions, the minimum water content gradually decreased. The crystal size distributions of plagioclase microlite in groundmass show large variations in small size (<1 µm) and these small crystals are inferred to crystallize at low pressure (<40 MPa), confirmed by decompression-induced crystallization experiments. Based on these observations, we infer the following scenario: the magma in the first sub-Plinian eruption ascended from the magma reservoir at the depth of ~4 km; however, the magma stagnated at a shallow conduit and then ascended to the surface during the second to the fourth sub-Plinian eruptions. Moreover, the position of the magma head in the conduit gradually became shallower. The results of this study indicate that the evolution of explosive eruptions during the 1977 eruption of Usu volcano originated from the processes and conditions of both the magma reservoir and the shallow parts of the conduit.