Earthworm can play a role in soil aggregate formation and soil carbon dynamics. However, their role in volcanic ash soil (Andosol), a dominant soil type in Japanese arable land, is poorly understood. We thus examined the effects of a common earthworm species, Eisenia japonica, on (1) soil aggregate size distribution and (2) soil carbon mineralization kinetics. In the first experiment, a surface layer of Andosol was 1-mm sieved and incubated for 3 weeks with or without earthworm and then we quantified aggregate size distribution by wet sieving. In second test, sieved soil (<1mm) with earthworm were incubated for 1 week to measure the concentrations of monosaccharides in soil solution and the mineralization kinetics of ¹⁴C-radiolabelled glucose in the macroaggregate (>1.6mm) formed by earthworm and in the bulk soil (<1mm). We found that earthworm increased the mass ratio of >2mm and <53µm fraction increased compared to the control soil without earthworm. The second experiment revealed that, while glucose concentration in soil solution was similar in both soils (30-34 μM), the mineralization of added glucose decreased by half in the aggregate soil (0.5 nmol g^- 1 soil h^-1) compared to the bulk soil. This study showed that earthworm promoted the >2mm aggregate formation and these macroaggregates reduced mineralization of added-glucose.