In a small saucepan, combine 1/2 cup of soy sauce, 1/2 cup of sugar, and 1/2 cup of mirin. Make sure to measure accurately for the best flavor.
Place the saucepan over medium heat. Stir the mixture gently with a whisk until the sugar dissolves completely. This should take about 2-3 minutes.
Once the sugar has dissolved, bring the mixture to a gentle simmer. Allow it to cook for an additional 5-7 minutes, stirring occasionally. You want the sauce to reduce slightly and thicken to a syrupy consistency.
Remove the saucepan from the heat and let the eel sauce cool to room temperature. Once cooled, transfer it to a glass jar or bottle and seal it tightly.