The effects of operating conditions on bubble volume formed from a nozzle submerged in some viscous media with yield stress were experimentally investigated. Pressure fluctuations in the gas chamber accompanied by bubble formation as well as bubble growth curves were measured. By analysis of the time course of pressure change in the gas chamber, the dependency of the gas flow rate into the gas chamber and the chamber volume on the polytropic coefficient of gas in the gas chamber was discussed. To simulate the bubble formation at a single nozzle in a viscous medium having yield stress, the non-spherical bubble formation model was proposed. The calculated bubble volumes agreed relatively well with the experimental ones.