Constituent minerals and water-soluble components of the volcanic ash discharged from the eruption of Mt. Motoshirane on January 23, 2018, were analyzed to investigate the source environment of this eruption. The ash sample included quartz, plagioclase, cristobalite, pyrite, alunite, kaolinite, and pyrophyllite; its mineral assemblage suggests that a high-temperature acid alteration zone had been formed in the volcanic edifice of Mt. Motoshirane. The presence of pyrophyllite in the ash sample indicates that the explosion of this eruption took place at a depth reaching the basement rocks of Mt. Motoshirane. Further, the adhesion amount of water-soluble components detected from the ash sample is smaller than that in the ashes from the 1982 eruption of Mt. Shirane, indicating that the ash discharge of the 2018 eruption of Mt. Motoshirane took place in a condition in which the degree of involvement of the liquid phase was relatively small.