A method of expression of a target foreign protein encoded by the structural gene region of a target foreign gene in a plant induced by copper ions, which comprises a step of activating with copper ions a transcription factor to induce activation of transcription of the target foreign gene by a region having a promoter function contained in the target foreign gene, wherein said target foreign gene is contained in a gene construct which is constructed so as to comprise (a) a target foreign gene comprising a region having a promoter function and a structural gene region encoding a target foreign protein; (b) a nucleotide sequence encoding a transcription factor by a foreign gene different from said target foreign gene; (c) an arbitrary promoter upstream from the nucleotide sequence encoding said transcription factor; and (d) a nucleotide sequence having a length of 48 bp or more from a 5'-untranslated region of a 130k/180k gene of tomato mosaic virus downstream from the region having a promoter function contained in the target foreign gene, or wherein (i) a first gene construct is constructed so as to comprise: (a) a target foreign gene comprising a region having a promoter function and a structural gene region encoding a target foreign protein; and (b) a nucleotide sequence haying a length of 48 bp or more from a 5'-untranslated region of a 130k/180k gene of tomato mosaic virus downstream from the region having a promoter function contained in the target foreign gene; and (ii) a second gene construct is constructed so as to comprise: (c) a nucleotide sequence encoding a transcription factor by a foreign gene different from said target foreign gene; and (d) an arbitrary promoter upstream from the nucleotide sequence encoding said transcription factor, and wherein the nucleotide sequence of said transcription factor encodes the yeast ACE1. said target foreign gene is contained in a gene construct which is constructed so as to comprise (a) a target foreign gene comprising a region having a promoter function and a structural gene region encoding a target foreign protein; (b) a nucleotide sequence encoding a transcription factor by a foreign gene different from said target foreign gene; (c) an arbitrary promoter upstream from the nucleotide sequence encoding said transcription factor; and (d) a nucleotide sequence having a length of 48 bp or more from a 5'-untranslated region of a 130k/180k gene of tomato mosaic virus downstream from the region having a promoter function contained in the target foreign gene, or wherein (a) a target foreign gene comprising a region having a promoter function and a structural gene region encoding a target foreign protein; (b) a nucleotide sequence encoding a transcription factor by a foreign gene different from said target foreign gene; (c) an arbitrary promoter upstream from the nucleotide sequence encoding said transcription factor; and (d) a nucleotide sequence having a length of 48 bp or more from a 5'-untranslated region of a 130k/180k gene of tomato mosaic virus downstream from the region having a promoter function contained in the target foreign gene, or wherein (i) a first gene construct is constructed so as to comprise: (a) a target foreign gene comprising a region having a promoter function and a structural gene region encoding a target foreign protein; and (b) a nucleotide sequence haying a length of 48 bp or more from a 5'-untranslated region of a 130k/180k gene of tomato mosaic virus downstream from the region having a promoter function contained in the target foreign gene; and (ii) a second gene construct is constructed so as to comprise: (c) a nucleotide sequence encoding a transcription factor by a foreign gene different from said target foreign gene; and (d) an arbitrary promoter upstream from the nucleotide sequence encoding said transcription factor, and wherein the nucleotide sequence of said transcription factor encodes the yeast ACE1. (i) a first gene construct is constructed so as to comprise: (a) a target foreign gene comprising a region having a promoter function and a structural gene region encoding a target foreign protein; and (b) a nucleotide sequence haying a length of 48 bp or more from a 5'-untranslated region of a 130k/180k gene of tomato mosaic virus downstream from the region having a promoter function contained in the target foreign gene; and (a) a target foreign gene comprising a region having a promoter function and a structural gene region encoding a target foreign protein; and (b) a nucleotide sequence haying a length of 48 bp or more from a 5'-untranslated region of a 130k/180k gene of tomato mosaic virus downstream from the region having a promoter function contained in the target foreign gene; and (ii) a second gene construct is constructed so as to comprise: (c) a nucleotide sequence encoding a transcription factor by a foreign gene different from said target foreign gene; and (d) an arbitrary promoter upstream from the nucleotide sequence encoding said transcription factor, and wherein the nucleotide sequence of said transcription factor encodes the yeast ACE1. (c) a nucleotide sequence encoding a transcription factor by a foreign gene different from said target foreign gene; and (d) an arbitrary promoter upstream from the nucleotide sequence encoding said transcription factor, and