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):(Chem 1)R2R1NCH2COCH2CH2COR3 (1)wherein R1 and R2 each independently represents a hydrogen atom, an alkyl group, an acyl group, an alkoxycarbonyl group, an aryl group, or an aralkyl group； R3 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.
