A monoclonal antibody or a functional fragment thereof that specifically recognizes one or more Siglec-15 polypeptides, which consists of an amino acid sequence described in any one of the following points (a) through (h) and inhibits the formation of osteoclasts and / or osteoclast bone resorption: (a) an amino acid sequence represented by SEQ ID NO: 2 in the Sequence Listing； (b) an amino acid sequence consisting of amino acid residues 21 to 328 of the amino acid sequence represented by SEQ ID NO: 2 in the Sequence Listing； (c) an amino acid sequence consisting of amino acid residues 1 to 260 of the amino acid sequence represented by SEQ ID NO: 2 in the Sequence Listing； (d) an amino acid sequence consisting of amino acid residues 21 to 260 of the amino acid sequence represented by SEQ ID NO: 2 in the Sequence Listing； (e) an amino acid sequence represented by SEQ ID NO: 4 in the Sequence Listing； (f) an amino acid sequence consisting of amino acid residues 21 to 341 of the amino acid sequence represented by SEQ ID NO: 4 in the Sequence Listing； (g) an amino acid sequence consisting of amino acid residues 1 to 258 of the amino acid sequence represented by SEQ ID NO: 4 in the Sequence Listing； and (h) an amino acid sequence consisting of amino acid residues 21 to 258 of the amino acid sequence represented by SEQ ID NO: 4 in the Sequence Listing.Un anticuerpo monoclonal o un fragmento funcional del mismo que reconoce específicamente uno o más polipéptidos de Siglec-15, que consiste en una secuencia de aminoácidos descrita en uno cualquiera de los siguientes puntos (a) a (h) e inhibe la formación de osteoclastos y/o la resorción ósea osteoclástica: (a) una secuencia de aminoácidos representada por SEC ID Nº: 2 en el Listado de secuencias； (b) una secuencia de aminoácidos que consiste en los restos de aminoácidos 21 a 328 de la secuencia de aminoácidos representada por SEC ID Nº: 2 en el Listado de secuencias； (c) una secuencia de aminoácidos que consiste en los resto
