The present invention relates to a Bacillus sp. strain producing a mannanase which is highly active in the neutral and the acidic media. The Bacillus sp. WL-1 strain (KCTC 0800BP), which is isolated from the soil, produces in large scale the mannanase in the culture medium containing lactose and bran, wherein the mannanase is highly active in the neutral and the acidic media so said mannanase is useful as an additive for feeds and is helpful in the decomposition of hemicellulose.