An insulating structure for an external terminal of an electronic part is formed of two insulating housing parts abutting along a junction to define an insulating housing for an electronic part. There is an insulating projection on each housing part and projecting outwardly of the housing from the corresponding housing part, each projection having a U-shaped cross-section with the open sides of the projections facing each other with one projection inside the other and the cross-sections telescopingly interfitted. One of the projections has a cross-section extending into the wall of the corresponding housing part from the junction and the other projection has the cross-section projecting from the junction toward the one projection. The wall of one of said housing parts has a first wall portion extending into the end of the corresponding projection where it extends from the housing part, and this wall portion has a recess therein facing toward the projection on the other housing part, and the wall of the other of the housing parts has a second wall portion extending into the end of the corresponding projection where it extends from the housing part and fitting into the recess in said first wall portion. The recess and second wall portion fit tightly around the terminal of the electronic part. A metallic casing around the insulating housing has an opening therein with the edges of the opening abutting the outside surface of the projections. Leakage current from the terminal to the metallic casing must traverse a long path through the compactly arranged projections.