Provided is a GSSG-containing fertilizer capable of enhancing the stability of oxidized glutathione GSSG in an environment such as soil and continuously exerting useful effects of GSSG on plants. The present invention relates to a sustained-release fertilizer containing GSSG and a release controlling agent.土壌中等の環境での酸化型グルタチオンGSSGの安定性を高め、植物に対するGSSGの有用な効果を持続的に発揮することが可能なGSSG含有肥料を提供する。 本発明はGSSGと、放出制御剤とを含む、徐放性肥料に関する。