This study deals with the improvement of a size-reduced in-pipe movable mechanism which will be employed to fabricate long and thin curved holes, the feasibility of electrical discharge machining by means of the improved mechanism and the extension of a curved hole by the mechanism. To establish a curved hole machining method, the authors planned to develop the long curved hole machining system which employs an in-pipe movable mechanism. Actually, the mechanism for machining a long curved hole has been developed. However, the diameter of its machining hole is too large. Therefore, the diameter of the mechanism has been reduced so that curved holes with the half diameter can be fabricated. However, the size-reduced mechanism has the problem that the shape of the fabricated holes is unstable. To overcome the problem, in the study, the structure of the mechanism is improved. From the results of the machining experiments, it is found that the improved mechanism can make a hole with performing a stable electrical discharge machining and has the ability of extending the curved hole length.