Tuberous roots of yacon (Polymnia sonchifolia) contain a large amount of oligofructans. So, they draw great attention as a healthful diet. Powder of tuberous roots and tea of leaves are commercially sold.Establishment of yacon cultivation technique is now a necessity. But the effect of topdressing on growth and yield of yacon has not yet been tested.Effects of topdressing were tested in 1993 and 1994.Results as follows:1) Tops of yacon grew vigorously and stem numbers increased by topdressing.2) Total yield of tuberous roots did not increase by topdressing.3) Yield of tuberous roots exceeding 300-400g decreased by topdressing.4) Topdressing has little effect on yield of tuberous roots in the tested.