A pressure-difference bubble transfer control method using a gas exchange apparatus comprising a gas exchange unit operable of performing gas exchange, a first flow channel being in contact with the gas exchange unit, and a second flow channel being in contact with the gas exchange unit and provided in such a manner that at least a part thereof is opposed to the first flow channel with the gas exchange unit interposed therebetween, wherein the method comprises: disposing the first flow channel for flowing a first fluid of liquid and the second flow channel for flowing a second fluid of liquid with the gas exchange unit, through which a liquid cannot pass but a gas component(s) can pass, being interposed therebetween; transferring a dissolved gas component(s) due to concentration gradient from the first fluid into the second fluid through the gas exchange unit, the dissolved gas component(s) being initially contained in the first fluid; and transferring bubbles from the second fluid into the first fluid through the gas exchange unit by making a pressure in the second flow channel higher than a pressure in the first flow channel, wherein the gas exchange unit has a plurality of grooves connecting the first and second flow channels to each other and not intersecting with one another, and wherein at least a part of the inner surface of each of the grooves is hydrophobized and the cross-sectional area of each of the grooves is determined so that a liquid cannot be transferred through the grooves but only a gas component(s) contained in the liquid can be transferred through the grooves, wherein a)the grooves are formed by a plurality of holes having a diameter of 1 μm or less, which penetrate a polycarbonate membrane, or b) the grooves have a width and a height of 10 μm or less and are formed in the surface of a first glass substrate, which faces the surface of a second glass substrate, in which the first and second flow channels are formed, or c) the grooves have a width and a height of 10 μm or less and are formed in the surface of a silicon substrate, in which the first and second flow channels are formed. disposing the first flow channel for flowing a first fluid of liquid and the second flow channel for flowing a second fluid of liquid with the gas exchange unit, through which a liquid cannot pass but a gas component(s) can pass, being interposed therebetween; transferring a dissolved gas component(s) due to concentration gradient from the first fluid into the second fluid through the gas exchange unit, the dissolved gas component(s) being initially contained in the first fluid; and transferring bubbles from the second fluid into the first fluid through the gas exchange unit by making a pressure in the second flow channel higher than a pressure in the first flow channel, wherein the gas exchange unit has a plurality of grooves connecting the first and second flow channels to each other and not intersecting with one another, and wherein at least a part of the inner surface of each of the grooves is hydrophobized and the cross-sectional area of each of the grooves is determined so that a liquid cannot be transferred through the grooves but only a gas component(s) contained in the liquid can be transferred through the grooves, wherein a)the grooves are formed by a plurality of holes having a diameter of 1 μm or less, which penetrate a polycarbonate membrane, or b) the grooves have a width and a height of 10 μm or less and are formed in the surface of a first glass substrate, which faces the surface of a second glass substrate, in which the first and second flow channels are formed, or c) the grooves have a width and a height of 10 μm or less and are formed in the surface of a silicon substrate, in which the first and second flow channels are formed. a)the grooves are formed by a plurality of holes having a diameter of 1 μm or less, which penetrate a polycarbonate membrane, or b) the grooves have a width and a height of 10 μm or less and are formed in the surface of a first glass substrate, which faces the surface of a second glass substrate, in which the first and second flow channels are formed, or c) the grooves have a width and a height of 10 μm or less and are formed in the surface of a silicon substrate, in which the first and second flow channels are formed.