PURPOSE: To provide the gate cutting mechanism of a mold for an optical disk not strictly requiring the concentricity of fixed and movable molds and producing burr at the time of the cutting of a disk.

CONSTITUTION: A gate cutting mechanism is equipped with the cylindrical die 1 arranged to the center part of one mold and the cutter 2 inserted in the die 1 and present at a standby position at the time of the filling with a resin and constituted so that the leading end surface thereof advances to an advance position immediately before the contact with a sprue bush after the filling with the resin and retreats to the retreat position indented from the end surface of the die. An undercut holding part 2a is formed to the leading end part of the cutter 2 and an undercut part is formed to the sprue 17 of a disk at the time of the solidification of the resin and held at the time of the retreat of the cutter 2 to take out the sprue 17 and the end part of the sprue 17 is cut by the edge of the die to perform gate cutting.