In a small saucepan, combine the ingredients over medium heat.
Stir the mixture until the sugar has dissolved and the sauce is well combined.
Bring the sauce to a simmer and let it cook for about 5 minutes, stirring occasionally.
Remove the sauce from heat and let it cool before transferring it to a jar or bottle for storage.
Notes
*1 if you can not access to Tonakatsu sauce, add 1 tbsp of Worcestershire sauce instead. *2 This recipe makes about 1/2 cup Yakisoba Sauce which is for three portions of yakisoba noodle.