Wash sweet potato and slice into 2cm thick, round shapes for easy eating.
Lay flat each pieces in pan or wide bas pot to avoid overlapping.
Put enough water to submerge each pieces. You may adjust later when needed.
Put sugar, mirin and soysauce and gently swirl the pan to mix well. Bring to a boil.
Once the liquid is half the height of potatoes , carefully flip each pieces to cook evenly.
Check the softness with a toothpick and may add more water if needed. Boil until all liquid has evaporated.