Involved in the degradation of glucomannan. Catalyzes the endo hydrolysis of beta-1,4-linked mannan, galactomannan and glucomannan. Random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans, galactomannans and glucomannans. kcat is 330 min(-1) for mannanase activity with galactomannan as substrate. kcat is 350 min(-1) for mannanase activity with glucomannan as substrate. Homodimer. Up-regulated by konjac glucomannan and by cellobiose and mannobiose, the possible degradation products of glucomannan. Repressed by glucose via the carbon catabolite repression system. Also repressed by GmuR. Belongs to the glycosyl hydrolase 26 family.