PURPOSE: To obtain chlorophyll a' or b' of high purity and photo-responce at a high yield and prevent the production of pheophytin, by dissolving chlorophyll a or b in an organic solvent which can form a ligand to magnesium located in the center of the chlorophyll a or b and by leaving the solution at a dark place.

CONSTITUTION: Chlorophyll a or b adjusted by a dioxane method is dissolved in a pure organic solvent such as pyridine or dioxane, which can form a ligand to magnesium located in the center of the chlorophyll. The solution is caused to perform reaction at room temperature, preferably 50W60°C, under an atmosphere of an inert gas such as argon in a dark place. The reaction solution is dried under reduced pressure, thereby obtaining a dark green substance. This substance is dissolved in a small quantity of petroleum ether and chlorophyll a' or b' is separated by sugar column chromatography. The chlorophyll a or b portion, which does not take part in the reaction, is used again for next reaction.