During a process of cooling a hypereutectic Al—Si alloy melt, ultrasonic vibration is applied to the melt to crystallize primary crystal α-Al using, in combination, an ultrasonic transducer (8) that generates the ultrasonic vibration, an ultrasonic horn (7) that is connected to the ultrasonic transducer (8) and transmits the ultrasonic vibration in a specified direction, a treatment vessel (2) that holds the melt and is in contact with the ultrasonic horn (7), and a treatment vessel fixing device (3) that fixes the treatment vessel (2) by pressing the treatment vessel toward the ultrasonic horn (7).