PROBLEM TO BE SOLVED: To provide a method and an apparatus both enabling light intensity corresponding to a light compensation point to be automatically gained irrespective of the kind of a seedling, ambient temperature, the growth stage of a seedling or the like. SOLUTION: This method comprises storing plant seedlings 2 in a low temperature chamber 1 where outside light and outside air are intercepted and plant seedlings 2 are illuminated by a lighting system 3, measuring the concentration of carbon dioxide in the chamber 1, controlling the lighting system so as for the inside of the chamber to become the state of a light compensation point, measuring the temperature inside the chamber and controlling the lighting system according to the temperature.
