Place mirin, sake, and sugar in a small saucepan.
Bring to a boil over medium-high; cook, stirring occasionally, until alcohol cooks off and sugar dissolves, about 2 minutes.
Stir in dashi and soy sauce; bring to boil.
Remove from heat.
Use the mentsuyu immediately for a warm dipping sauce, or let cool until room temperature, about 30 minutes.
Cover and store in refrigerator for up to 1 week.
