an insulation film whose requisite constituent elements are first elements and a second element . the first elements include b , p , o and fe . the second element can generate cations whose hexa - coordinated ion radius , defined by shannon , r . d ., is 0 . 073 nm or more , and which are bivalent or more . since the second element having a large ion radius is incorporated into network formers made from the first elements , it is possible to improve the heat resistance of the insulation film .