The present invention provides a method for specifically detecting a Mycobacterium tuberculosis complex-specific secretory protein MPT64 antigen in a biological sample, whereby diagnosis of infection with Mycobacterium tuberculosis is carried out rapidly and safely with higher accuracy than before. An antibody that recognizes an epitope for MPB64 located in any one of amino acid sequences of SEQ ID NOS: 2 to 4, particularly a monoclonal antibody was obtained. Thus, an immunoassay using the antibody, particularly a sandwich immunoassay using first and second antibodies to MPB64, particularly an immunochromatographic assay and an immunochromatographic test strip are provided. A biological sample can be rapidly subjected to the immunoassay without culturing or after culturing for a time before Mycobacterium tuberculosis complex bacteria in the sample substantially start to grow. The biological sample may be pretreated by treatment for inactivation of Mycobacterium tuberculosis, or treatment by dispersion or solubilization.
