To investigate the mechanism of yield increase by elevated carbon dioxide (CO₂) and fogging in Japanese tomato cultivars (Solanum lycopersicum), total above-ground dry matter (TDM), fraction of dry matter distribution to fruit (FDF), and photosynthetic characteristics were measured in 3 Japanese cultivars grown in elevated CO₂ with fogging and ambient CO₂ without fogging. Fresh fruit yield and TDM were improved by the elevated CO₂ and fogging in the 3 Japanese cultivars. Light use efficiency (LUE) was also increased by the elevated CO₂ and fogging. No significant decrease in FDF was observed by the elevated CO₂ and fogging in 2 Japanese cultivars, ‘Asabiyori 10’ and ‘Junkei Aichi Fast’. Thus, the increase in TDM by higher LUE contributed directly to the yield increase in these 2 cultivars. However, FDF in ‘Momotaro York’ was decreased significantly by the elevated CO₂ and fogging. Thereby, the yield increase by the elevated CO₂ and fogging was diminished in ‘Momotaro York’ in spite of the increase in TDM. The number of trusses having immature fruit in ‘Momotaro York’ under elevated CO₂ and fogging was significantly higher than those of the others, although no increase in the number of trusses having immature fruit was observed in the other 2 cultivars. Although vegetative growth characteristics such as leaf area, LAI, and fresh and dry weights of leaves and stem were increased by the elevated CO₂ and fogging, no negative effects such as a change in light-extinction coefficient and a decrease in maximum photosynthetic rate were observed. The elevated CO₂ and fogging increased the number of harvested fruit but decreased weight per fruit, namely, fruit size, in the 3 cultivars.