To provide a polyphenol-content-increasing agent which increases the content of polyphenol in a plant. A polyphenol-content-increasing agent for plant, containing, as an active ingredient, a 5-aminolevulinic acid or a derivative thereof represented by the following formula (1): €ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ(Chem 1) R 2 R 1 NCH 2 COCH 2 CH 2 COR 3 €ƒ€ƒ€ƒ€ƒ€ƒ(1) wherein R 1 and R 2 each independently represents a hydrogen atom, an alkyl group, an acyl group, an alkoxycarbonyl group, an aryl group, or an aralkyl group； R 3 represents a hydroxyl group, an alkoxy group which may have a substituent, an acyloxy group, an alkoxycarbonyloxy group, an aryloxy group, an aralkyloxy group, or an amino group, or a salt thereof.
