A reaction apparatus comprises a first supply flow channel having a fine flow channel cross sectional area for a liquid pressurized by a pressurizing device, a gas supply flow channel having a fine flow channel cross sectional area for supplying a gas, a two-phase flow channel having a fine flow channel cross sectional area in communication with a joined portion for the first supply flow channel and the second supply flow channel for flowing a gas/liquid two-phase fluid, a gas bubble reaction flow channel in communication with the exit of the gas/liquid two-phase channel and having a flow channel cross sectional area larger than that of the gas/liquid two-phase flow channel, and a liquid discharge flow channel for discharging the liquid in the gas bubble reaction flow channel. Therefore, in the reaction apparatus, a stable mixing ratio of a gas to a liquid can be obtained and the mixing speed of the gas to the liquid is increased.