A natriuretic peptide receptor GC-A agonist for use in preventing or suppressing the metastasis of a malignant tumor, wherein the natriuretic peptide receptor GC-A agonist is any one selected from the following to or a pharmacologically acceptable salt thereof and has an agonist activity for the natriuretic peptide receptor GC-A: an atrial natriuretic peptide consisting of the amino acid sequence of SEQ ID NO: 1 or 2, a brain natriuretic peptide consisting of the amino acid sequence of SEQ ID NO: 3, 4, or 5, a substance comprising an active fragment, wherein it has the amino acid sequence of SEQ ID NO: 6, or wherein it consists of the amino acid sequence from position 7 to position 27 of SEQ ID NO: 1 or 2, the amino acid sequence from position 10 to position 30 of SEQ ID NO: 3 or 4, or the amino acid sequence from position 23 to position 43 of SEQ ID NO: 5, a mutant wherein it is (i) a peptide consisting of the amino acid sequence of SEQ ID NO: 1 or 2 having substitution, deletion, insertion, and/or addition of one to several amino acids at any one to several positions selected from positions 1 to 6 and 28, (ii) a peptide consisting of the amino acid sequence of SEQ ID NO: 3 or 4 having substitution, deletion, insertion, and/or addition of one to several amino acids at any one to several positions selected from positions 1 to 9,31 and 32, (iii) a peptide consisting of the amino acid sequence of SEQ ID NO: 5 having substitution, deletion, insertion, and/or addition of one to several amino acids at any one to several positions selected from positions 1 to 22, 44 and 45; a derivative comprising any one of the amino acid sequences of SEQ ID NOs: 1 to 5, wherein the derivative preferably further comprises at least one of the Fc region of an immunoglobulin, a serum albumin, and the C-terminus of ghrelin, and a modified form wherein it comprises any one of the amino acid sequences of SEQ ID NOs: 1 to 5 of the sequence listing, and at least one amino acid other than the amino acids shown in SEQ ID NO: 6 is chemically modified, or wherein it is prepared by chemical modification by addition of a pharmaceutically usable polymer. an atrial natriuretic peptide consisting of the amino acid sequence of SEQ ID NO: 1 or 2, a brain natriuretic peptide consisting of the amino acid sequence of SEQ ID NO: 3, 4, or 5, a substance comprising an active fragment, wherein it has the amino acid sequence of SEQ ID NO: 6, or wherein it consists of the amino acid sequence from position 7 to position 27 of SEQ ID NO: 1 or 2, the amino acid sequence from position 10 to position 30 of SEQ ID NO: 3 or 4, or the amino acid sequence from position 23 to position 43 of SEQ ID NO: 5, a mutant wherein it is (i) a peptide consisting of the amino acid sequence of SEQ ID NO: 1 or 2 having substitution, deletion, insertion, and/or addition of one to several amino acids at any one to several positions selected from positions 1 to 6 and 28, (ii) a peptide consisting of the amino acid sequence of SEQ ID NO: 3 or 4 having substitution, deletion, insertion, and/or addition of one to several amino acids at any one to several positions selected from positions 1 to 9,31 and 32, (iii) a peptide consisting of the amino acid sequence of SEQ ID NO: 5 having substitution, deletion, insertion, and/or addition of one to several amino acids at any one to several positions selected from positions 1 to 22, 44 and 45; (i) a peptide consisting of the amino acid sequence of SEQ ID NO: 1 or 2 having substitution, deletion, insertion, and/or addition of one to several amino acids at any one to several positions selected from positions 1 to 6 and 28, (ii) a peptide consisting of the amino acid sequence of SEQ ID NO: 3 or 4 having substitution, deletion, insertion, and/or addition of one to several amino acids at any one to several positions selected from positions 1 to 9,31 and 32, (iii) a peptide consisting of the amino acid sequence of SEQ ID NO: 5 having substitution, deletion, insertion, and/or addition of one to several amino acids at any one to several positions selected from positions 1 to 22, 44 and 45; a derivative comprising any one of the amino acid sequences of SEQ ID NOs: 1 to 5, wherein the derivative preferably further comprises at least one of the Fc region of an immunoglobulin, a serum albumin, and the C-terminus of ghrelin, and a modified form wherein it comprises any one of the amino acid sequences of SEQ ID NOs: 1 to 5 of the sequence listing, and at least one amino acid other than the amino acids shown in SEQ ID NO: 6 is chemically modified, or wherein it is prepared by chemical modification by addition of a pharmaceutically usable polymer.