Tuberous roots of yacon (Polymnia sonchifolia) contain a large amount of oligofructans. So they draw great attention as a healthful diet. Processed food of tuberous roots and tea of foliages are commercially sold already. But reports on cultivation of yacon are few, influence of planting depth of seed tubers on growth and yield not yet tested. Tested were the planting depth of seed tubers in ridges 25cm high.Three planting depths: 5cm, 10cm and 15cm were tested. Each plot had three replications.Results as follows:1) Growth of main stem was least in the 5cm plot.2) Top dry weight per plant was 196.4g, 215.3g and 223.5g in 5cm, 10cm and 15cm plots respectively.3) Yield of tuberous roots per plant was 1413.0g, 1525.7g and 1269.6g in 5cm, 10cm and 15cm plots respectively. Yield of tuberous roots over 100g per plant was 970.1g, 1142.0g and 914.2g in each plots.4) Percentage of tuberous roots over 300g was least in the 15cm plot.5) Gaseous phase ratio at planting depth at harvest time was 55.08%, 47.14% and 38.17% in 5cm, 10cm and 15cm plots respectively.6) In ridges 25cm high, 10cm should be the suitable planting depth for yacon seed tubers.