PURPOSE: To enable accurate detection of mold reducing rate by fitting a dolly bar as changeable toward approaching or alienating direction to a casting slab at nearer side to the casting slab than to a non-contacting type position detector fitted to a reducing mold body block.

CONSTITUTION: The position detectors 32, 33, which detect the reducing rate of the casting slab 2 at the side faces of the reducing mold body block 14, 16 and the dolly bars 34, 35 are fitted apart from the detector 32, 33 under the described distance y1. When the dolly bard 34, 35 are brought into contact with the casting slab 2 by decending and ascending of each block 14, 16, the dolly bars 34, 35 are gradually deformed to become to narrow distance y1, and next the distance y2, at the time of contacting the blocks 14, 16 with the casting slab 2, is detected by the detectors 32, 33. After that, even if, the blocks 14, 16 reduces the casting slab 2, the dolly bars 34, 35 are not shifted and the distance y3 becomes gradually shorter and the reducing rate yR=y2-y3 is detected continuously. In this way, the reducing rate of the mold to be casting slab is accurately detected and the service life of the position detector is lengthened.