We studied the suitability of a PCR method using samples of skin and whole blood and serological tests for the early diagnosis of Japan spotted fever (JSF) in its acute and convalescent stages and compared the advantages and disadvantages of these different diagnostic methods. In the acute stage, the percentage of positive results was 91.2 % for the PCR method using skin samples, 52.3 % for the PCR method using whole blood samples, and 40.4 % for the serological tests with IgM. In the convalescent stage, paired serum showed positive results (IgM, 98.5 %; IgG, 94.0 %). [corrected]. The PCR method using samples of skin (eschar) is the most sensitive, specific, and suitable method for promptly and accurately diagnosing JSF in the early stage. Therefore, this method is recommended for early definite diagnosis of JSF in the critical stage.