A center mechanism of a tire vulcanizer in accordance with the present invention has an upper clamp ring for holding the upper opening end of a bladder, a lower clamp ring for holding the lower opening end of the bladder, a lower ring on the lower mold side supporting the lower clamp ring, a spacer installed to the lower ring and having a ventilating portion, and a center post elevating cylinder installed to the spacer. The extending portion of piston rod of the center post elevating cylinder is made a center post, the center post is inserted, in such a manner as to be movable vertically, into a through hole formed in the center of the lower ring and the lower clamp ring, and the upper clamp ring is fixed to the upper end portion of center post protruding upward from the through hole of the lower clamp ring.