The flow characteristics in the bubble column, in the case of highly viscous liquid, were experimentally examined. Carboxymethyl cellulose (CMC) aqueous solution was used as the liquid phase, and the liquid height, column diameter, gas velocity and CMC concentration were changed. The flow pattern was observed, and the gas holdup and volumetric mass transfer coefficient were measured. When a large bubble, or slug, flowed out of the liquid surface, the liquid surface was violently disturbed and small bubbles were entrained into the liquid. The entrained small bubbles had a significant influence on the flow characteristics. The gas holdup and volumetric mass transfer coefficient decreased with increasing liquid height and were unchanged beyond a certain liquid height. The critical aspect ratio of the column was about 8, independently of the column diameter, gas velocity and liquid viscosity. Experimental equations on the volumetric coefficient were obtained.