|In Japan、 river flow data related to medium sized and small rivers are scarce compared to similar resources for major rivers - a situation attributable to the current problem of inefficient river flow measurement. Accordingly、 a laborsaving、 safe and reliable measurement method should be established. The radio current meter is expected to enable safe、 automatic flood-discharge monitoring because of its suitability for unmanned flow rate measurement without being in contact with the water itself. The authors applied velocity correction to measurements taken with the radio current meter using the ratio of surface velocity to cross-sectional mean velocity、 and examined a method for smooth switching of the sectional area of the stream before and after flooding. The ratio of surface velocity measured using a radio current meter to cross-sectional mean velocity measured using an ADCP was found to be almost equal to the surface float coefficient (0.85). It was also confirmed that cross-sectional mean velocity could be evaluated using surface velocity with an accuracy of approximately ±10% even when this coefficient was applied as the velocity correction factor. A cross-sectional area of flow close to that shown by actual measurement values from the start to the end of flooding was reproduced by estimating the time-series change in the river channel cross section using friction velocity values found from continuous flow velocity data obtained with a radio current meter and by smoothly switching the cross sectional area of flow from that before to that after flooding.