An under-cover mounting structure for an industrial vehicle includes right and left side-frames, and an under cover. The right and left side-frames face each other with an interval. The under cover is detachably mounted on the side frames by fixing means for covering a lower part of an area between the side frames. The under cover has right and left side-portions. At least one of the side-portions has a mounting inclined-surface formed by folding the under cover along a longitudinal direction thereof. At least one of the side frames has a mounted inclined-surface which faces the mounting inclined-surface of the under cover.