In a wire-cut EDM apparatus for cutting a workpiece (2) submerged in a processing tank (3) filled with a machining fluid (15), an opening (24) is made in the side wall of the tank through which an arm unit passes. A slide plate (14) also has an opening for passage of the arm unit and also provents leakage from the space between the opening 24 and the arm unit. In one preferred embodiment a fluid flow between the slide plate and arm unit is used to prevent fluid in the tank from escaping through the space between the slide plate (14) and the arm unit. Alternatively, a low frictional member may be placed between the arm unit and the slide plate for axially sealing the arm unit. Frictional resistance from movement of the lower arm unit is reduced and machining accuracy improved.