A coupling position switching mechanism capable of improving efficiency of a coupling position switching operation, while realizing structural simplification, and reductions in size and cost. A first coupling portion consisting of one of a slide metal frame-side coupling portion and a drive unit-side coupling portion is formed with a groove-shaped recess and a through-hole in this order from the side of a distal end of the first coupling portion, and a second coupling portion consisting of the remaining one of the slide metal frame-side coupling portion and the drive unit-side coupling portion is formed with the through-hole. The first and second coupling portions are configured such that, during a casting operation, they are coupled together by a coupling pin inserted into the through-hole of the first coupling portion and the through-hole of the second coupling portion, and during surface pressure-applying/releasing operation, they are coupled together by a second coupling pin inserted into the groove-shaped recess of the first coupling portion and the through-hole of the second coupling portion.