PURPOSE: To constantly obtain a laser disk substrate with excellent flatness.

CONSTITUTION: (a) indicates the condition several seconds before mold release. Mold release air valves of both force side part 1a and cavity side 1b are turned on (for supply) for mold release, and a specified air pressure is applied to the air blow slits in the molds. (b) indicates the condition immediately after the mold release. Mold release begins, and when the distance between a fixed platen 3a and a movable platen 3b become a specified one from the fully closed condition, a signal is put out from a mold release detection circuit 5 and the condition in which the mold release air valve of the cavity side is closed is indicated. The mold release air of the cavity side may be shut off within 0.00 (simultaneously with the mold release) -0.02 second from the start of mold release.