PURPOSE: To improve stability of an organic substrate substance to light without impairing hue and purity of the organic substrate substance, especially organic dye or organic pigment, etc., by blending the organic substrate substance with a (dipyridyl)(cis-1,2-ethylene dithiolato) nickel derivative.

CONSTITUTION: cis-1,2-Ethylene dithiol derivative or alkali metal salt thereof is dissolved in an anhydrous methanol and a (dipyridyl)(dihalo) nickel derivative complex is added to the solution as a powder or methanol solution and stirred and the resultant precipitate is filtered, extracted with an organic solvent and concentrated and methanol is added to the concentrate to provide the (dipyridyl)(cis-1,2-ethylenedithiolato) nickel derivative expressed by the formula (R1WR4 are H, halogen, CN, OH, alkyl, aryl, etc.; R5 and R6 are H, alkyl, alkylthio, aryl, etc.), which is then blended with an organic substrate substance (e.g. quinone imine dye or polyolefin) to stabilize the substance.