When a reaction liquid is passed through a gas-liquid separation pipe having a larger volume per unit length than the volume per unit length of a pipe, a headspace part into which gas can be released is formed above the gas-liquid separation pipe, and the byproduct gas generated by the reaction changes into bubbles, moves upward in the reaction liquid, and is continuously released from a gas-liquid interface into the headspace part. While the gas is removed in the gas-liquid separation pipe, the pressure of the headspace part is controlled so as to be constant by a pressure adjustment device.