The first wave of the coronavirus disease 2019 (COVID-19) epidemic occurred between March 2020 and May 2020 in Japan. The first epidemic wave in Chiba prefecture was analyzed based on the case series reported by the official health department of Chiba prefecture. The mean interval from the onset of symptoms to a definitive diagnosis by PCR was 6.9 days. The number of cases of infection, onset of symptoms, and of definitive diagnosis peaked on March 31, April 2 and April 10, respectively. The effective reproduction number was below 1.0 in February, 2.0 to 3.0 between March 17 and March 27, and decreased to under 1.0 again after April 2, 2020. Some patients with an infectious duration of more than 10 days showed an effective reproduction number in the 3.0 to 4.0 range at the peak of the first epidemic.