The present invention relates to a method for producing optically active IHOG, which can in turn be used for the production of monatin. The present invention further relates to a method for producing optically active monatin, and aldolase used for these methods. As such, the present invention enables the synthesis of 4-(Indole-3-ylmethyl)-4-hydroxy-2-oxoglutaric acid with high optical purity, which is useful as an intermediate in the synthesis of optically active monatin, from indole pyruvic acid and pyruvic acid (or oxaloacetic acid).