An object of the present invention is to provide a detection marker that can simply and rapidly detect Mycoplasma pneumoniae, which is a pathogen of mycoplasma pneumonia, at a high sensitivity, a specific antibody against the marker, and also an immunological detection method and a kit containing the antibody.Infection with Mycoplasma pneumoniae can be rapidly and specifically diagnosed by producing an antibody specifically reactive to P30 protein of Mycoplasma pneumoniae and performing an immunological assay using the P30 protein as a detection marker. The present invention enables easy and rapid detection of Mycoplasma pneumoniae and diagnosis of infection with the bacteria at a hospital or the like without need of specialized instruments or skilled techniques.