Conophylline, contained in leaves of Ervatamia microphylla or Tabernaemontana divaricata, can induce non-tumor cells to differentiate into insulin-producing cells and thereby to produce insulin and also to lower blood glucose levels. Thus, in the present invention, dried products and extracts of leaves of Ervatamia microphylla or Tabernaemontana divaricata, which contain conophylline, are used as health foods for preventing and improving diabetes and obesity or health foods for lowering blood glucose levels.