Whisk together JUST egg, soy sauce, mirin, and sugar.
Heat a tamagoyaki pan* over medium low. Brush some of the oil on the surface of the pan.
To test if the pan is hot, add a small drop of the egg mixture to the pan. If it sizzles, the pan is ready.
Pour a thin layer of the egg mixture into the pan and tilt the pan to coat the bottom.
Cook for about a minute, until the bottom has set and the top is just barely cooked.
Carefully roll the egg up into a log shape (it's easiest to do this with a spatula). Move the egg to the far side of the pan and re-oil the pan.
Pour another thin layer of egg in and repeat the process until all the egg mixture is used up (I usually make about 4 layers).
Transfer the tamagoyaki to a plate or cutting board. To help set the shape, you can wrap the tamagoyaki in a bamboo sushi mat while hot and let it rest for 5 minutes.