A camera for capturing a casting bead is provided at each side end of a discharge port in a width direction thereof. Plural speakers are provided at the vicinity of an opening of a decompression chamber. The camera continuously captures the casting bead, and outputs image data thereof to a control unit. The control unit includes a vibration detector and a suppressing vibration generator. The vibration detector detects vibration of the casting bead based on the plural image data. The suppressing vibration generator generates suppressing vibration signal representing the phase and amplitude of the suppressing vibration for suppressing the vibration generated in the thickness direction of the casting bead. The control unit controls the speakers based on the suppressing vibration signal to output sound wave. Thereby, the suppressing vibration is applied to the casting bead.