Catalyzes the endo hydrolysis of beta-1,4-linked mannan, galactomannan and glucomannan. It is able to hydrolyze mannosidic linkages that are flanked by mannose or glucose. Random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans, galactomannans and glucomannans. kcat is 140000 min(-1) for mannanase activity with galactomannan as substrate. kcat is 62000 min(-1) for mannanase activity with glucomannan as substrate. Belongs to the glycosyl hydrolase 5 (cellulase A) family.