After phosphate depletion in a batch culture medium, the growth rate of Ajuga hairy root decreased and the content of a secondary metabolite, 20-hydroxyecdysone (20-HE), increased, The 20-HE content in Ajuga hairy Foot increased when the intracellular phosphate content was kept less than 5 mg/g-dry weight. Indoleacetic acid (IAA) supplementation (0.1 mg/l) increased the growth rate of Ajuga hairy root due to an increase in the number of root apical meristems although it led to a decrease in 20-HE content. From the above results, the culture was divided into two phases : a growth phase where all nutrient components in the medium are present above the critical level, and a secondary metabolite production phase where intracellular phosphate content is kept less than 5 mg/g-dry weight. A kinetic model was constructed to simulate the intracellular phosphate content during the culture. The fed-batch culture in which 0.1 mg/l of IAA was supplemented only at the beginning of the culture, the intracellular phosphate content was kept below 5 mg/g-dry weight after 10 d, and then phosphate was eliminated from the medium after 20 d, was carried out in a turbine-blade reactor by controlling the medium condition according to the conductivity of the culture medium and the simulation data. The culture provided approximately 3-fold increase in 20-HE content, compared to the fed-batch culture without phosphate control.