Abstract: Vibration of a jet pump installed in a reactor pressure vessel of a boiling water reactor is monitored. Ultrasonic waves are transmitted from an ultrasonic sensor installed on an outer surface of the reactor pressure vessel toward a throat and a diffuser of the jet pump. When the ultrasonic waves reach respective outer surfaces of the throat and diffuser, reflected waves are generated at the respective outer surfaces. The ultrasonic sensor receives those reflected waves. The ultrasonic signal process section obtains a distance in the horizontal direction between the throat and the diffuser based on a time difference of the reflected waves reflected from respective reflection surfaces of the throat and diffuser and a sound speed in reactor water in the neighborhood of the throat and diffuser. A relative vibration is obtained based on the change with time of the distance.