To specify the epicentres of 118 disastrous earthquakes occurred in Japan in history, we adopted an angular variable θ as shown in Fig. 1. Plotting each earthquake in a rectangular coordinate-system, the ordinate and abscissa of which denote era and θ respectively, and taking the weighted mean of θ for every 30 years, we obtained the centres of activity as shown in Fig. 2, where the weights of the most disastrous, disastrous and rather disastrous earthquakes are taken to be 3, 2 and 1 respectively. The θ's of these centres are found to be well arranged in a form θ=θ1+θ2 where and θ2 is also a periodic function which changes its value zero or a series of positive ones at an interval of 66 years. The maximum of θ2 is about 15°. (cf. Fig. 3 and 4).