PURPOSE: To manufacture a device at a low cost without need for setting an expensive sensor at all times while improve operation efficiency of or assembling and maintenance by fitting a rotor measuring sensor into a sensor inserting hole as necessary.

CONSTITUTION: A cover 14 is removably fitted in a sensor inserting hole 1a on the side face of a casing 1. And, an annular projection made of a vacuum retainable material such as a rubber, etc. is provided on the cover 14 to maintain the vacuum of the inside of the casing 1. Accordingly, since a sensor is not installed in a casing 1, there is no need of removing wiring at the time of removing the casing 1 for the repair, inspection, etc. of the inside. Thereby, a maintenance or assembling operation can be simply carried out, greatly improving operation efficiency. Also, when measuring the unsteadiness of a rotor 3, measuring can be carried out merely by fitting a sensor into the sensor inserting hole 1a. Thus, by eliminating the need for setting an expensive rotor measuring sensor at all times, a device can be produced at a low cost.