To find out an in-vitro bioassay system capable of ensuring qualities of yokukansan to a higher degree, it is intended to provide a method of bioassaying yokukansan characterized by comprising adding a yokukansan-containing test sample to astroglial cells to be cultivated under the condition of thiamine deficiency, and then determining the pharmacological activity value of yokukansan based on the glutamic acid or neutral red intake level in the cultivated astroglial cells.