A device (20) for assembling a plastic-lens-forming casting mold includes a gasket clamping mechanism (22) which clamps a gasket (2) from its axial direction, a first mold push-in mechanism (23) which pushes in an upper mold (3) fitted in an upper open end of the gasket (2), and a second mold push-in mechanism (24) which pushes in a lower mold (4) fitted in a lower open end of the gasket (2). The gasket clamping mechanism (22) clamps the gasket (2) from the axial direction by a gasket-pressing ring (34) and lower mold ring (35). The first mold push-in mechanism (23) pushes the upper mold (3) into the gasket (2) by a gasket push-in member (58). The second mold push-in mechanism (24) includes a plurality of push pins (68) and pushes up the lower mold (4) to push it into the gasket (2).