PURPOSE: To control selectively and effectively the cell activity, e.g. respiration activity, by applying a given voltage generated in an optical semiconductor by irradiation with light directly to cells.

CONSTITUTION: Powder of an optical semiconductor, e.g. InAs, having a relatively high electron-positive hole mobility ratio, is added to a suspension prepared by suspending bacteria, actinomyces, molds, microalgae, yeasts, erythrocytes, leukocytes, tumorous cells, etc. in a phosphoric acid buffer solution to give a dispersion 2 of the dispersed optical semiconductor powder to be treated. The resultant dispersion 2 to be treated is then put into a water tank 1 and irradiated with light under agitation with an agitation means 3 to apply a given voltage generated in the optical semiconductor directly to the cells.