Provided are a method of producing a filling which improves water-retention capability during heating conducted after the filling is wrapped, and which has an excellent texture, and a filled food product produced by using the filling. Specifically, provided is a filling for a filled food product, the filling comprising at least one or more heat-processed filling ingredients and water-soluble cellulose ether which is gelable during heating. Also provided is a method of producing a filling for a filled food product comprising at least the steps of: heat-processing one or more filling ingredients; and mixing the heat-processed filling ingredients with water-soluble cellulose ether which is gelable during heating, to thereby obtain a filling. Further provided is a filled food product obtained by performing any one of heating, baking, and frying of: a filling comprising at least one or more heat-processed filling ingredients and water-soluble cellulose ether which is gelable during heating; and a dough skin which wraps the filling.