First, gather all the ingredients in separate bowls or cups.
In a saucepan, pour and mix the sake and mirin.
Next, add the soy sauce and stir.
Add the sugar and stir well until it is dissolved.
Turn on the heat to medium and bring all the ingredients to a boil. Keep stirring until the sugar dissolves completely.
Lower the heat to medium-low and simmer for about 12 to 15 minutes. At this point, the sauce should be thickened.
Small bubbles will start to rise to the surface as you stir the sauce or tilt the pan. The sauce is then prepared for usage at this point.
Put the sauce in a clean jar and let it cool there. As it cools, the sauce will get thicker. You can serve it once cooled.