A slide piece driving mechanism includes a side surface guiding portion, a shaft portion, and stopper releasing element. The side surface guiding portion is disposed at a movable side mold to guide a side surface of a slide piece to be slidably movable. The shaft portion of a stopper is disposed at the side surface guiding portion. When the slide piece reaches an upper end position, the shaft portion engages with a stopper hole of the slide piece to prevent the slide piece from falling from the upper end position due to own weight. The stopper releasing element releases the engagement of the shaft portion of the stopper with the stopper hole to allow the slide piece from falling from the upper end position due to its own weight.