A solid composition that controls the decrease of reduced coenzyme Q10 due to its oxidation during storage is described. The solid composition contains a co-crystal of reduced coenzyme Q10, an emulsifier, and an antioxidant. Additionally, a method for producing a particulate solid composition containing a co-crystal of reduced coenzyme Q10, an emulsifier, an antioxidant, and a binder is described. The method includes granulating a mixture containing the co-crystal of reduced coenzyme Q10, the emulsifier, the antioxidant, the binder, and a liquid binder.