Abstract Annual rings record the intensity of cosmic rays (CRs) that had entered into the Earth’s atmosphere. Several rapid 14 C increases in the past, such as the 775 CE and 994CE 14 C spikes, have been reported to originate from extreme solar proton events (SPEs). Another rapid 14 C increase, also known as the ca. 660 BCE event in German oak tree rings as well as increases of 10 Be and 36 Cl in ice cores, was presumed similar to the 775 CE event; however, as the 14 C increase of approximately 10‰ in 660 BCE had taken a rather longer rise time of 3–4 years as compared to that of the 775 CE event, the occurrence could not be simply associated to an extreme SPE. In this study, to elucidate the rapid increase in 14 C concentrations in tree rings around 660 BCE, we have precisely measured the 14 C concentrations of earlywoods and latewoods inside the annual rings of Japanese cedar for the period 669–633 BCE. Based on the feature of 14 C production rate calculated from the fine measured profile of the 14 C concentrations, we found that the 14 C rapid increase occurred within 665–663.5 BCE, and that duration of 14 C production describing the event is distributed from one month to 41 months. The possibility of occurrence of consecutive SPEs over up to three years is offered.