PURPOSE: To exactly detect a liquid level by providing a movable body following the change of the level of liquid for cooling in a rotor and a photosensor detecting its floating level.

CONSTITUTION: Light is cast from a lighting device 12 to a liquid level detector inside a rotor with using a light casting photofiber 13 for leveling in 4 for example. At the liquid detection level, a moving body 11 following the change of the liquid level 5 is provided. If the liquid level in the rotor is at F, the upper two lights from the photofiber 13 are blocked and lower two lights are transmitted through the photofibers 15 for light reception to the level 1 detector 14a and the level 2 detector 14b in the light reception device 14, respectively, and thus, the liquid level is detected. Following the lowering of the liquid level, the movable body 11 also lowers and the light is not transmitted in the detector 14b and the detector 14a in this order and thus, the lowering of the liquid level is judged. When the liquid level 5 rises, conversely, the movable body 1 rises, the light is transmitted in the level 3 detector 14c and the level 4 detector 14d in this order, and the rise of the liquid level 5 is judged.