Paenibacillus sp. DZ3, newly isolated from Konjack field, produced ${eta}$-mannanase (900 U/mL) when grown on glucomannan as a carbon source. ${eta}$-Mannanase was purified 34-fold to homogeneity resulting in final recovery of 15% and specificity of 169 U/mg protein. The molecular mass was approximately 39 kDa as estimated by sodiumdodecylsulfate-polyacrylamide gel electrophoresis. Active band was observed as clear colourless area on zymogram. The optimal temperature and pH for mannanase activity was $60^{circ}C$ and pH 5.0, respectively. The activity was stable up to $60^{circ}C$ at pH 5.0 and remained stable from pH 5.0-7.0. Mannanase was highly specific towards glucomannan and galactomannan, whereas exhibited low activity towards mushroom powder. The Michaelis constant ($K_m$) and maximum velocity ($V_{max}$) for glucomannan substrate were 1.05 mg/mL and 714 U/mg, respectively. These results indicate the enzyme is attractive for industrial applications.