A gas/liquid mixing device is provided in which bubbles having such a small diameter as to be invisible to the eye are produced and dissolved in water in a short time, whereby the solubility of gas can be improved. The gas/liquid mixing device has a mixing cylinder, a drive unit, and a gas supply device. In the mixing cylinder, a rotary vane having guide vanes is rotated to compress water in the mixing cylinder and gas supplied into the mixing cylinder so that the water flow containing the bubbles is dispersed outwardly through bubble dispersing holes formed in the wall of the mixing cylinder.