The purpose of the present study was to evaluate the accuracy and usefulness of three rapid diagnostic methods, ImmunoCard Mycoplasma kit, chest high-resolution computed tomography (HRCT) findings, and the Japanese Respiratory Society (JRS) scoring system (including six parameters), for the early presumptive diagnosis of Mycoplasma pneumoniae pneumonia in adults. We performed three rapid diagnostic methods at the same time in four pneumonia groups: 68 cases with M. pneumoniae pneumonia, 133 cases with Streptococcus pneumoniae pneumonia, 30 cases with Haemophilus influenzae pneumonia, and 20 cases with Legionella pneumonia. The sensitivity and specificity were 35% and 68% for ImmunoCard, 73% and 85% with HRCT, and 83% and 90% with the JRS scoring system, respectively. Among the three rapid diagnostic methods, the JRS scoring system was the most useful tool for initiating the administration of adequate antibiotic therapy for probable M. pneumoniae pneumonia. We suggest that M. pneumoniae pneumonia should be suspected when there is a correlation of more than five parameters in the JRS scoring system (99% specificity). If there is a correlation of three or four parameters in the JRS scoring system, chest computed tomography (CT) findings are helpful for the presumptive diagnosis of M. pneumoniae pneumonia.