We evaluated the usefulness of a sputum antigen detection kit for Streptococcus pneumoniae in a rapid diagnosis in 166 adult patients (113 males and 53 females) with community-acquired pneumonia who simultaneously underwent all 4 tests of sputum Gram stain, sputum culture, urinary antigen detection test, and sputum antigen detection test in 14 sites in Japan. A total of 53 patients (31.9%) were sputum culture-positive for S. pneumoniae. Based on the sputum culture results, the sensitivity for detecting S. pneumoniae was 73.6% in sputum Gram stain, 62.3% in urinary antigen detection test, and 88.7% in sputum antigen detection test. The sensitivity in the sputum antigen detection test was significantly higher than that in the urinary antigen detection test in patients with mild infection (p<0.03) and in patients whose symptoms started within 2 days before the tests (p<0.03). Sputum antigen detection tests showed a significantly higher concordance rate than the sputum Gram stain tests (p<0.01). These results suggest that the sputum antigen detection kit is useful in the rapid diagnosis of community-acquired pneumococcal pneumonia in adults.