The present invention relates to a novel antibody specifically binding to human and mouse L1CAM, and more particularly, to an antibody binding to both human and mouse L1CAM with high affinity, which is prepared by modifying a sequence of an L1 cell adhesion molecule (L1CAM)-specific antibody comprising a heavy-chain variable region of SEQ ID NO. 1 and a light-chain variable region of SEQ ID NO. 5, a polynucleotide encoding the antibody, an expression vector comprising the polynucleotide, a transformant introduced with the vector, a pharmaceutical composition for preventing or treating cancer comprising the antibody, a method for treating cancer using the antibody, a composition for diagnosing cancer comprising the antibody, a kit for diagnosing cancer comprising the composition, a method for providing information for cancer diagnosis using the antibody, and an antibody-drug conjugate prepared by conjugating a drug to the antibody.