A natural flavor or fragrance can be easily and effectively produced by using a “unicellularized plant”, obtained by unicellularizing, without destroying, the cells of a plant usable as a starting material for a natural flavor or fragrance, as a material for the natural flavor or fragrance and further applying an enzymatic treatment or chemical treatment or continuously heating the same, to thereby remarkably shorten the aging period for producing the natural flavor or fragrance, or by homogenizing a plant usable as the starting material quickly after harvesting to thereby promote the aging.