A polyphenol-content-increasing agent which increases the content of a polyphenol in a plant. The polyphenol-content-increasing agent for plants is characterized by containing as an active ingredient any of 5-aminolevulinic acid or a derivative thereof represented by the following general formula (1): (Chemical formula 1) RRNCHCOCHCHCOR (1) (wherein R and R each independently represents hydrogen, alkyl, acyl, alkoxycarbonyl, aryl, or aralkyl； and R represents hydroxy, optionally substituted alkoxy, acyloxy, alkoxycarbonyloxy, aryloxy, aralkyloxy, or amino) and salts of these.
