To provide a cellulose composite powder which is especially excellent in heat stability and acid resistance and gives an aqueous dispersion excellent in stability by compounding cellulose, karaya gum, and a dispersant so that the resultant product has a specified relaxation time.

This composite powder is prepared by subjecting a cellulose material, karaya gum, and a dispersant to wet milling and drying and comprises 50-94 wt.% cellulose having an average base particle size of 20 μm or lower, 5-49 wt.% karaya gum, and 1-30 wt.% dispersant. When this powder is heated at 100°C for about 4-20 hr to adjust the water content to 8.0±0.5 wt.% and is then subjected to the measurement of transverse relaxation time T2 of 1H nucleus by using the solid echo method by pulse NMR equivalent to 25 MHz at 30°C, the value of relaxation time T2,L obtained by the optimization calculation with the relation formula of time t and magnetization Mt, represented by the formula (wherein M0 is the magnetization when the relaxation is almost completed; and T2,G, T2,L, XG, and M0 are each an optimization coefficient used for optimization calculation), is 400-480 μs.