In a medium saucepan, bring sake to a boil over moderate heat. Let the alcohol evaporate; this will take about 30 seconds.
Add the soy sauce, mirin, kombu, and dried bonito flakes (katsuobushi).
Bring everything to a rolling boil, then simmer on low heat for 5 minutes. Turn off the heat, and set the sauce aside until it cools down. Once cool, strain the sauce and set aside. (This sauce keeps well in an airtight container, refrigerated, for up to a month.)