We examine the decreasing of seismic activity which occurred in Shikoku, and the Wakayama and Tanba districts prior the 1946 Nankai earthquake, southwest Japan. Considering the effect of the Second World War, the seismic observation for that period by the Japan Meteorological Agency was not in sufficient conditions. Thus, the idea of seismicity lowering on the area has not been generally accepted, notwithstanding some studies on the subject were previously presented. This study proves however the lowering was not caused by the insufficient seismic observation. We give an explanation for the cause of lowering of seismicity. We also showed that the crustal seismic activity in Shikoku in terms of earthquake occurrence gradually decreased with time after the 1946 earthquake. The decrease in seismicity could be anticipated to continue till the next Nankai earthquake in the above mentioned districts. Accordingly, the lowering of seismic activity is expected as a precursor of the great Nankai earthquake, in particular in the Wakayama district where the usual seismic activity is comparatively high compared to other districts.