We developed a rapid and sensitive diagnosis system for H5N1 highly pathogenic avian influenza (HPAI) virus infection using an unique gene amplification method, reverse transcriptase loop-mediated isothermal amplification (RT-LAMP). The sensitivity of the system was found to be 100-fold higher than that of ordinary one-step RT-PCR. Moreover, by using viral RNAs extracted from influenza viruses of all 15 HA subtypes, the RT-LAMP system was confirmed to amplify only the RNA of H5 subtype virus. In the surveillance of H5N1 virus infection of wild birds, we detected two positive cases from dead crows found near the affected area with H5N1-HPAI by using RT-LAMP system, although one of two positive cases was missed by RT-PCR. These results suggested that our newly developed RT-LAMP system specific for H5 virus would be a beneficial diagnostic tool for surveillance of recent outbreaks caused by H5N1-HPAI viruses.