This study confirmed a utility of a 10HS sensor, a capacitance sensor, in converted paddy fields to exploit the practical influence volume from our results and the corrected FSC function. Converted paddy fields in Japan are typically used to cultivate lowland rice or upland crops, and many of them have a problem of poor drainage when upland crops are cultivated. One of the major causes of poor drainage is a presence of a hardpan layer due to soil puddling for lowland rice cultivation. To monitor soil moisture above the hardpan layer without the influence of the hardpan layer, we identified the spatial sensitivity around the sensor and determined the applicability of the sensor with the factory-supplied calibration (FSC) function, including under near-saturated conditions. As a result, regarding the spatial sensitivity, the influence volume of the sensor from our result is much smaller than that reported by Cobos (2008), and we suggested that the sensors should be installed at least 1.5 cm above the boundary between the hardpan and plowed layers. Moreover, regarding the calibrations of the sensors, a corrected FSC function that can account for the bulk density was proposed.