Disclosed is a mold attaching/detaching apparatus in a tire vulcanizer, having locking/unlocking means configured so as to perform an engagement and disengagement by utilizing the relative movement between a top plate and bottom plate of the tire vulcanizer in a horizontal direction, and capable of being inexpensively constructed by simplifying the structure and operation of the locking/unlocking means. In the tire vulcanizer configured so that a segmental mold 3 is held between the top plate 1 and the bottom plate 2 to perform vulcanization, and opened and closed while expanding and contracting the segments by raising and lowering the top plate and operating an expansion/contraction device 4, in an engaged state of the locking/unlocking means 50 and 61, the locking/unlocking means is engaged in a state where the center of the top plate and the center of the bottom plate coincide with each other at a vulcanizing position M by the relative movement between the top plate and the bottom plate in the horizontal direction, and is disengaged at an offset position N shifted from the vulcanizing position in the horizontal direction.