The effects of low pressure on biomass production, flowering and fruiting of dwarf tomato were investigated. Partial pressures of O-2, CO2 and N-2 could be controlled separately. Continuous observation of growth of plant materials was possible through acrylic windows fitted into the chamber body. Dwarf tomato 'Regina' was grown under three conditions, 30 kPa and 50 kPa (hypobaric) and 100 kPa (standard atmospheric pressure; control). The whole plant biomass production of plants grown under 50 kPa was larger than that of plants grown under other pressure. This greater biomass production was due to an increase in CO2 gas diffusivity under low pressure environment and an increase in photosynthetic rate. Lower whole plant growth under 30 kPa compared with other pressures was due to a decrease in relative humidity caused by lowering the pressure using a vacuum pump. The number of flower clusters did not differ among the three treatments, while the number of flowers under the 50 kPa condition increased significantly compared with other treatments. Tomato plants are day-neutral, so the promotion of growth at 50 kPa also promoted flower formation.