A natriuretic peptide receptor GC-B agonist for use in preventing or suppressing the metastasis of a malignant tumor, wherein the natriuretic peptide receptor GC-B 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-B: a c-type natriuretic peptide consisting of the amino acid sequence of SEQ ID NO: 7, 8, 9 or 10, a substance comprising an active fragment, wherein it has the amino acid sequence of SEQ ID NO: 11, or wherein it consists of the amino acid sequence from position 6 to position 22 of SEQ ID NO: 7, a mutant wherein it is (i) a peptide consisting of the amino acid sequence of SEQ ID NO: 7, 9 or 10 having substitution, deletion, insertion, and/or addition of one to several amino acids selected from positions 1 to 5, (ii) a peptide consisting of the amino acid sequence of SEQ ID NO: 8 having substitution, deletion, insertion, and/or addition of one to several amino acids at any one to several positions selected from positions 1 to 36, a derivative comprising any one of the amino acid sequences of SEQ ID Nos: 7 to 11, 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: 7 to 10 of the sequence listing, and at least one amino acid other than the amino acids shown in SEQ ID NO: 11 is chemically modified, or wherein it is prepared by chemical modification by addition of a pharmaceutically usable polymer. a c-type natriuretic peptide consisting of the amino acid sequence of SEQ ID NO: 7, 8, 9 or 10, a substance comprising an active fragment, wherein it has the amino acid sequence of SEQ ID NO: 11, or wherein it consists of the amino acid sequence from position 6 to position 22 of SEQ ID NO: 7, a mutant wherein it is (i) a peptide consisting of the amino acid sequence of SEQ ID NO: 7, 9 or 10 having substitution, deletion, insertion, and/or addition of one to several amino acids selected from positions 1 to 5, (ii) a peptide consisting of the amino acid sequence of SEQ ID NO: 8 having substitution, deletion, insertion, and/or addition of one to several amino acids at any one to several positions selected from positions 1 to 36, (i) a peptide consisting of the amino acid sequence of SEQ ID NO: 7, 9 or 10 having substitution, deletion, insertion, and/or addition of one to several amino acids selected from positions 1 to 5, (ii) a peptide consisting of the amino acid sequence of SEQ ID NO: 8 having substitution, deletion, insertion, and/or addition of one to several amino acids at any one to several positions selected from positions 1 to 36, a derivative comprising any one of the amino acid sequences of SEQ ID Nos: 7 to 11, 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: 7 to 10 of the sequence listing, and at least one amino acid other than the amino acids shown in SEQ ID NO: 11 is chemically modified, or wherein it is prepared by chemical modification by addition of a pharmaceutically usable polymer.