The invention relates to compounds which are useful as orally administrable drugs (particularly, therapeutic or preventive agents for influenza) having sialidase−inhibiting activity and the guanidino groups of which are protected with specific acyl groups. The compounds are represented by the general formula (1): R<sp>x</sp>−NHC(=NR<sp>10</sp>)NHR<sp>11</sp> (1) wherein R<sp>x</sp> is a residue of a compound (1A) represented by the general formula: R<sp>x</sp>−NHC(=NH)NH<sb>2</sb> and having sialidase −inhibiting activity; and R<sp>10</sp> and R<sp>11</sp> are each hydrogen, −C(=O)OC(R<sp>G1</sp>)− (R<sp>G2</sp>)OC(=O)R<sp>G3</sp>, −C(=O)OC(R<sp>G1</sp>)(R<sp>G2</sp>)OC(=O)OR<sp>G4</sp>, or −C(=O)OR<sp>G5</sp> (wherein R<sp>G1</sp>, R<sp>G2</sp>, R<sp>G3</sp>, and R<sp>G4</sp> are each hydrogen, alkyl, or the like), with the proviso that the cases wherein both R<sp>10</sp> and R<sp>11</sp> are hydrogen are excepted.
