A movable platen support structure includes: a linear guide mechanism via which a movable platen is provided for horizontal movement; a plurality of movable-platen height adjustment mechanisms provided in corresponding relation to leg sections of the platen, each of the height adjustment mechanisms including a flat plate placed on a corresponding slider of the guide mechanism, and an adjusting bolt screwed into the corresponding leg section of the slider and abutting at its distal end against the flat plate; and a plurality of fastening mechanisms provided in corresponding relation to the leg sections, each of the fastening mechanisms including a fastening bolt for, following the height adjustment by the adjusting bolts, fastening the corresponding leg section to the corresponding slider by being passed through the leg section and the flat plate so as to be screwed into the slider.