The conditions for production of high cell density of Bifidobacterium longum were investigated and the cross-flow filtration system was used to remove the inhibitory metabolites, lactic acid and acetic acid. The maximum cell growth was observed with glucose as carbon source at the concentration of 50 g/ℓ at 37℃ with the initial pH 6.5. When B. longum was cultured in a cross-flow filtration system, the maximum cell growth was observed at a dilution rate(D) of 0.31 h^(-1) and the dry cell weight was 16.4 g/ℓ(3.5×10^(10) cell/㎖), which was about four times higher than that obtained in the batch culture with pH control.