PURPOSE: To discover the abnormality in solidified shell formation in an early time by providing a sensor case sandwiching the three faces including the corners of an ingot in a U-shape, taking out the temps. of the ingot surface as light energy, and quantifying the solidification state in the corner parts.

CONSTITUTION: Optical fibers F1WF56 are connected to optical fiber rods FR1W FR56 enclosing the corners of an ingot 2 in a U-shape, and are bundled to one piece in a sensor 1, thence these are introduced into an optical scanner 100. The energy radiated from the ingot 2 is put into a rotary disc 103 by the fibers FR1WFR56, then through an optical fiber 110 into an optical branching filter 120, by which it is branched to two beams of light. The two color temp. signal SA of an aithmetic circuit 125 is inputted to an expansion circuit 128. On the other hand, a detection signal is fed to a start detector 126 at every detection of a mark M by a detector D, and the address signal (i) of an address forming circuit 127 is inputted to the circuit 128. Thereby, a two dimensional graph is developed and the surface temp. distribution of the three surfaces of the sensor 1 sandwiching the corners of the ingot 2 is monitored.