A blade vibration monitoring device includes: a rotary machine including a rotating shaft that extends along an axis, and a plurality of moving blades having moving blade bodies that extend radially outward in a radial direction from the rotating shaft, and shrouds that are provided at tips of the moving blade bodies and are in contact with each other in a circumferential direction; and a sensor that is provided on the outside of the shroud in the radial direction to face the shroud, and is configured to detect a change in an outer circumferential surface of the shroud, in which the outer circumferential surface of the shroud includes a first surface, and a second surface which is disposed to be interposed between the first surfaces from both sides in the circumferential direction, and in which a detection signal from the sensor is different from that of the first surface.