In an expansion valve having a control mechanism with an airtight chamber, the airtight chamber is sealed by a steel ball joined into a filling hole in a metallic wall of the airtight chamber. The steel ball is fixed by resistance welding for sealing the filling hole. The diameter ratio between the inner diameter of the filling hole and the outer diameter of the spherical surface of the steel ball is set to a value falling in a range from 0.6 to 0.85.