To make it possible to correctly determine whether the detected ultrasonic signal is a true reflected wave signal or a noise signal when obtaining the ratio of a solidified layer (the solidus rate) by detecting the reflected wave from a boundary surface between the solidified layer and a non-solidified layer of a material to be cast during the casting.

The relationship between the temperature of a molten metal 31 and the ultrasonic velocity in the molten metal is obtained. The ultrasonic wave is irradiated in a cavity to detect the first reflected wave of the ultrasonic wave returned from a die surface 2a and the second reflected wave of the ultrasonic wave returned from a die surface 2b. The ultrasonic wave propagation time (t) from the detection time of the first reflected wave to the detection time of the second reflected wave. The ultrasonic wave velocity (v1) immediately before solidification and the ultrasonic wave velocity (v2) immediately after solidification of the molten metal are obtained from the relationship between the temperature of the molten metal and the ultrasonic wave velocity. The solidus rate of the molten metal is calculated based on the ultrasonic wave propagation time (t), and the ultrasonic wave velocity immediately before and immediately after solidification. The threshold level Th is set to the detection level of the second reflected wave, and the die 2 is controlled by using the solidus rate of the molten metal calculated when the detection level is higher than the threshold level.