By modifying a previous simple rapid method, we propose an in-situ simple, rapid calibration method based on soil porosity and the highest raw sensor output (RAW) obtained after heavy rainfall during the monitoring period. According to the results of our verifying experiments for three soils, we concluded that the modified method is applicable to monitoring results, including previously obtained uncalibrated data, that satisfy three criteria: (1) the monitoring data must include at least a few peak RAW values of around 1,200-1,300, (2) the monitoring data must capture changes in the water content at appropriate intervals, and (3) it must be possible to estimate the soil porosity of the soil at the monitoring site.