A microalga isolated from Sagami Bay in Japan was cultivated in a tubular reactor and its productivity was obtained, The alga productivity indicated the maximum value at the initial algae concentrations of nearby 0.6g/ l and was influenced by the flow rate. It was compared with those simulated based on basic research. Relatively good correlations between the actual productivities and the simulated ones were found and a flow rate dependency on productivities was confirmed. Utilization studies of the alga as a pulp additive indicated its effectiveness for anti-printthrough, smoothness and tolerance for deterioration, and the alga could therefore be a possible pulp substitute.