Beat together the eggs, mirin, soy sauce and sugar until well combined.
Heat 1/3 of the oil in a non-stick frypan over medium heat. Add 1/3 of the egg mixture, making sure it evenly covers the bottom of the pan. Once the omelette has just set, start rolling it up like a sausage from one edge.
Keep the rolled egg to one side of the pan and add another 1/3 of the egg mixture, evenly covering the pan as before. Once set, roll the cooked egg roll back the other way, adding another layer of egg to the sausage.
Repeat with the last of the egg mixture and roll it up as before. Transfer to a bamboo sushi rolling mat, roll up tightly and let cool for a few moments. Unwrap and cut into six slices.