PURPOSE: To perform oxidation and reduction effectively by providing a gas dissolving part having a gas-liquid mixing part and a screw flow path member, a gas-liquid mixing dissolver having an excess gas separating vessel and a compressor feeding the pressurized gas to the dissolver, etc.

CONSTITUTION: When air contg. a large quantity of oxygen of the almost same pressure and water are fed to a gas-liquid introducing cylinder part 111 in a gas- liquid mixing dissolver 100 before feeding raw material to a pressurizing-floatation-separation vessel 200, both the two are mixed and sent into a gas dissolving part B via fluid holes 113a. Since a screw flow path is formed in the inside of an external cylindrical body 112a of the gas dissolving part B, a long flow path is obtained in the short straight distance and the mixing-dissolving process is made long and thereby the mixing-dissolving efficiency is increased. Also, since the respective screw groove members consist of plural streaks, the flow path is divided into small area and useful to the improvement of the mixing-dissolving efficiency.