Heat and mass are transferred by natural convection in the ponded water of paddy fields. For a detailed study on convective fields in ponded water, we devised an improved, constant current, hot-wire method to measure the convection flow in a ponded paddy field. A long constantan wire was used as the hot wire, and two nickel (Ni) wires were used for sensing. One of the Ni wires was attached to the constantan wire to detect the temperature of the constantan wire on heating. Another Ni wire was used to detect the temperature of ambient water. These Ni wires were connected to a bridge circuit so that the output of the circuit would indicate the temperature difference between the hot wire and the ambient water. The temperature rise in the hot wire was dependent on both the power supply and the convection flow in the water. In this study, a 10-mW cm−1heat source was found to be experimentally effective. For calibration, the temperature rise was measured in stationary water by moving the sensor at very low velocities. The temperature rise decreased as the sensor velocity increased. The temperature rise was inversely proportional to the convective velocity.The convective velocity measured in a paddy field by this sensor was extremely low and fluctuated with time. The temperature difference between the boundaries of the water layer, the driving force for natural convection in the field, shows a daily fluctuation depending on the local weather conditions. The maximum velocity was 0.14 cm s−1at around noon, corresponding to the maximum temperature difference. This sensor can be employed to measure convective velocity in the field; however, a manual check of the field measurements and calibration before and after use of the sensor in the field are necessary.