To provide a method and a device for determining the concentration by which the effect of a growth-modifying agent on the growth of a target microorganism can be recognized.

This method comprises (a) a process for preparing a culture medium for growth of the microorganism, (b) a process for preparing a detective reagent containing the growth-modifying agent mixed with a marker (the signal of the strength proportional to the concentration of the marker is exhibited), (c) a process for integrating the detective reagent into the culture medium for growth so as to form the concentration gradient between the growth-modifying agent and the marker in the culture medium for growth, (d) a process for inoculating the target microorganism into the culture medium for growth, (e) a process for incubating the inoculated culture medium for growth during the time needed to be increased up to the amount capable of detecting the target microorganism, (f) a process for evaluating the growth specific characteristic of the microorganism in a domain containing the growth-modifying agent, (g) a process for measuring the strength of the marker signal in the domain and (h) a process for determining the concentration of the growth-modifying agent by the use of the measured strength of the marker signal.