To achieve the miniaturization of a gas-liquid separator of a fine air bubble supply device without lowering a gas-liquid separation capacity.

The gas-liquid separator of the fine air bubble supply device includes a pressure container (11) which has an introducing port (14) for introducing a dissolving liquid from a gas dissolver and a lead-out port (15) for leading out the dissolving liquid formed in its lower part, and also has an exhaust valve (18) attached to its upper part, and a partition member (17) which is provided in the pressure container (11), formed over the range from the base of the pressure container (11) to the upper part in the pressure container (11) and partitioning the inside of the pressure container (11) into an introducing space (S1) on the side of the introducing port (14) and a lead-out space (S2) on the side of the lead-out port (15). The partition member (17) is inclined so that the cross-sectional area of the introducing space (S1) becomes large as going toward the upper part.