Place the mirin in a pot or deep pan and heat it over medium heat. When the boiling bubbles get bigger, the alcohol has evaporated.
Reduce the heat low, add the sugar and stir gently with a rice paddle to prevent it from clumping at the bottom of the pot.
Once the sugar has dissolved, add the soy sauce.
Remove the white scum and simmer over low heat for about 10 minutes. Be careful not to let it boil, as the flavor and aroma of the soy sauce will be lost.
Cool down: Remove the saucepan from heat and let the kaeshi cool to room temperature.
Transfer the cooled kaeshi to a clean, airtight container.
Notes
*1 Read the types of Soy Sauce in the complete guide to Soy sauce post.*2 More precisely if you have a scale, weigh 15g of sugar.