A method of protecting a carbonaceous crucible by interposing a protective sheet made of an expanded graphite material between a quartz crucible and a carbonaceous crucible on which the quartz crucible is placed, comprising: installing two or more protective sheet pieces on an inner surface of the carbonaceous crucible, each of the protective sheet pieces having such a shape that the protective sheet pieces are combined integrally to form the protective sheet in installing the protective sheet between the crucibles, wherein: each of the protective sheet pieces has securing means for securing the protective sheet pieces that are adjacent to each other in a condition in which the protective sheet pieces are installed on the inner surface of the carbonaceous crucible, the securing means comprising a configuration in which a protruding portion is provided on an end part of one of the protective sheet pieces that are adjacent to each other while a recessed portion in which the protruding portion fits is provided in an end part of another protective sheet piece, so that the protective sheet can be installed by inserting the protruding portion into the recessed portion, or a configuration in which a cut is formed in an end of one of the protective sheet pieces, and another protective sheet piece is inserted into the cut. installing two or more protective sheet pieces on an inner surface of the carbonaceous crucible, each of the protective sheet pieces having such a shape that the protective sheet pieces are combined integrally to form the protective sheet in installing the protective sheet between the crucibles, wherein: each of the protective sheet pieces has securing means for securing the protective sheet pieces that are adjacent to each other in a condition in which the protective sheet pieces are installed on the inner surface of the carbonaceous crucible, the securing means comprising a configuration in which a protruding portion is provided on an end part of one of the protective sheet pieces that are adjacent to each other while a recessed portion in which the protruding portion fits is provided in an end part of another protective sheet piece, so that the protective sheet can be installed by inserting the protruding portion into the recessed portion, or a configuration in which a cut is formed in an end of one of the protective sheet pieces, and another protective sheet piece is inserted into the cut. each of the protective sheet pieces has securing means for securing the protective sheet pieces that are adjacent to each other in a condition in which the protective sheet pieces are installed on the inner surface of the carbonaceous crucible, the securing means comprising a configuration in which a protruding portion is provided on an end part of one of the protective sheet pieces that are adjacent to each other while a recessed portion in which the protruding portion fits is provided in an end part of another protective sheet piece, so that the protective sheet can be installed by inserting the protruding portion into the recessed portion, or a configuration in which a cut is formed in an end of one of the protective sheet pieces, and another protective sheet piece is inserted into the cut.