This paper reports our current development of "Robo-ducky", the rice field weeding robots, which is inspired from rice farming with natural ducks. This paper presents (1) the improvement of motion ability of the robot by modifying the size of the robots, and (2) the investigation of weeding ability of the robot and damage put to rice plants in an agricultural scientific way such as counting up the number of seeds up from soil and defect rice plants according to the number of robot weeding. These investigations show the robot has enough weeding ability in actual rice fields.