PURPOSE: To detect gas leakage from a shaft seal part quantitatively.
CONSTITUTION: Near a driving device 6, there is a gas sensor 2 which detects gas leakage from a cabin 9. An air suction vane 1 which directs the air near the cabin 9 to the driving device forcibly between the driving device of a rotating shaft 7 and the cabin 9. Moreover, a cylindrical windshield 4 is installed so as to surround the rotating shaft 7, the air suction vane 1 and the gas sensor 2, kept between the cabin 9 and the driving device 6. An arithmetic processing device 3 inputs an analyzing value signal which shows gas concentration from the gas sensor 2, and a signal which shows the rotating speed of the rotating shaft 7 from the driving device 6, and computes gas leakage amount = ventilating amount x gas concentration.