The 2021 eruption of Fukutoku-Oka-no-Ba (FOB), which is a submarine volcano located at the southern end of the Izu-Ogasawara arc, produced a large number of pumice clasts that drifted to many places in the islands of Japan and eastern Asia. Amongst the typical gray pumice clasts, several peculiar clasts have been discovered, such as those with a black coloration and containing mafic enclaves. This study found a mostly bimineralic enclave consisting of plagioclase phenocrysts and an alkali feldspar matrix, with minor cristobalite, TiO2 minerals (anatase and rutile), and Fe sulfide. The chemical composition of the plagioclase phenocrysts is similar to that reported from the FOB pumice, and the tie line of the alkali feldspar and plagioclase in a Ca-Na-K ternary diagram indicates that they originated from melt extracted from the crystal mush of the FOB magma reservoir. The cristobalite occurs in the voids in the matrix, in which surrounding alkali feldspar compositions changed gradually along the ternary feldspar solvus of ∼ 850 °C. The formation of a cristobalite-bearing bimineralic enclave can be explained by (1) the melt was extracted and accumulated at the shallow part of the magma reservoir, which crystallized as syenitic rocks; (2) subsequent degassing-related alteration within the volcanic conduit that caused plagioclase breakdown and cristobalite crystallization; and (3) entrainment of the syenitic rock fragment by the nanolite-bearing magma being erupted from the conduit.