PURPOSE: To adopt each advantage of longitudinal wave and transverse wave ultrasonic methods and to enable measurement at high accuracy under overcoming the defect by using the measured positions and ultrasonic propagating time by each one pair of the transverse wave ultrasonic transceivers arranged near the solidified position of the cast slab and at downstream side thereof.

CONSTITUTION: Each one pair of the transverse wave ultrasonic transceivers 2, 4 arranged as inserting the cast slab 1 near solidified position of the cast slab 1 and at downstream side thereof are scanned while synchronizing to the width direction of the cast slab 1. Position of the solidified condition which becomes the reference having the prescribed solid phase ratio by whether the penetrating signal of the transceiver 2 is on or not, is detected and also the ultrasonic propagating time in the cast slab 1 is measured by the transceiver 4. From these values, a parameter showing relation between preset ultrasonic propagating time and distance from the transceiver 4 to the aimed solidified ratio point is corrected. Then, distance to the point having the aimed solidified ratio from ultrasonic propagating time at each point is measured and the solidified profile is constituted and measured from these value.