A dissolving apparatus has a cylindrical tubular container closed at both ends with its center axis being inclined to the horizon. A center of an interface between a gas and a liquid in the container is positioned at a center in a lengthwise direction of a side wall of the container. Two inner spaces of the container above and below the interface are referred to as a gas section and a liquid section. An injection inlet for injecting a gas-liquid mixed fluid into the container is provided at a level corresponding to, or lower than, the interface. A liquid outlet for discharging the liquid is provided near a bottom of the liquid section of the container. Since the container is inclined, the interface can have an area large enough to promote dissolution of the gas into the liquid. Since the depth of the liquid in the liquid section is sufficiently deep, the liquid can be prevented from being discharged through the liquid outlet with large gas bubbles being present therein.