This paper describes the design optimization of the structure of a tool for producing curved holes by electrochemical machining and the results of machining experiments. Curved holes can be machined using a flexible tool that can be curved by the hole being processed by the tool itself. The tool for producing small holes of less than 6 mm diameter has a simple structure composed of a flexible tube and a columnar electrode tip. A tool device equipped with an ultrasonic vibration function was designed and fabricated to remove the sludge from the curved face of the hole during machining. Machining tests showed that the machining speed was able to exceed the target value of 1mm/min.