To ensure the device maintainability in emergency at low cost and keep the reliability of a gained data by providing a projection on the moving blade disc side of a rotation traverse disc, and providing stoppers nipping the projection with a space on the moving blade disc.

The relative positions of a moving blade disc 2 and a rotation traverse disc 5 are generally determined within the range of the space 11 between a projection 6 and stoppers 4 by the step motor of a circumferential positioning device and a control method thereof. Namely, the both are moved within the space while keeping a prescribed phase difference, and synchronously rotated to perform a measurement. When a failure such as interruption of the power source of the step motor takes place, the relative positions of both the discs 2, 5 are freely changeable, but the projection 6 is collided with the stoppers 4 to prevent them from being slipped over a prescribed range. Thus, the measurement data is never made unusable, and can be safely used.