A method of manufacturing an organic silicon compound that contains a methacryloxy group or an acryloxy group, the method being characterized by the fact that manufacturing or conducting purification by distillation is carried out in the presence of a phenothiazine derivative having a molecular weight equal to or higher than 240. And a stable composition comprising an organic silicon compound that contains a methacryloxy group or an acryloxy group and a phenothiazine derivative having a molecular weight equal to or higher than 240 and used in an amount sufficient to stabilize the aforementioned organic silicon compound.