A flexible pouch is filled with fluid food contents at a filling ratio such that in a graph showing the relationship between sterilization time and head space gas amount wherein the axis of ordinate and the axis of abscissas represent the sterilization time and the head space gas amount, respectively, the sterilization time becomes constant without dependence upon the head space gas amount. The pouch after the filling is hermetically sealed and subjected to sliding sterilization in nonbound condition.
