The present invention provides a novel method of manufacturing germinated plant seeds suitable for producing a large amount of phytoalexin, a method of manufacturing raw material seeds for germination induction for use in manufacture of the above germinated plant seeds, an extract composition of the germinated plant seeds and a screening method for a plant seed candidate for use in producing a target substance. The method of manufacturing raw material seeds for germination induction comprises a pre-treatment step of maintaining plant seeds under atmosphere conditions of a carbon dioxide concentration of 400 ppm or more and/or an oxygen concentration of 19 vol % or less continuously for 5 hours or more.