Abstract: In this application are disclosed a fruit- or berry-quality improving agent containing proline as the effective ingredient, which agent is in the form of an aqueous solution of proline with a 50% or higher purity and having a concentration ranging from 15 to 1,500 ppm in terms of the proline, and a method of improving fruits or berries in quality which method comprises spraying an aqueous solution of proline with a 50% or higher purity and having a concentration ranging from 15 to 1,500 ppm in terms of the proline onto the leaf surfaces of a fruiting plant at the fruit or berry growing period or over the period between the flower-bud formation period or before and the fruit or berry growing period. According to the present invention are provided an excellent agent for improving fruits or berries in quality and a method of improving fruits or berries in quality with the use of the same.