A method and an apparatus for dissolving a gas into and mixing the same with a liquid is provided for mixing bubbles with and dispersing the same into a liquid and/or for dissolving a gas into a liquid efficiently. The apparatus comprises a throttled section (12) formed in a fluid flow passage, a gas inlet (18) provided slightly downstream of the throttled section (12), a widened section (16) continuous to the throttled section (12), a mixing section (20) provided downstream of the widened section (16), for mixing the gas introduced from the gas inlet (18) with the liquid in the fluid flow passage, and a nozzle section (24) provided on the outlet of the mixing section (20). The method is achieved by using this apparatus. The method and the apparatus are applicable to methods and apparatuses for dissolving ozone into a liquid, for dispersing bubbles in a liquid and floating small particles in the liquid, and the like methods and apparatuses.