The invention enables an abnormality of a temperature sensor to be simply detected.
In S1, a temperature measurement is performed with each temperature sensor, and measured signals are digitalized to obtain a temperature value. Then, in S2, a detection temperature Tb′ equivalent to a detected temperature of a corresponding temperature sensor is calculated with use of an arithmetic expression and a characterization factor stored in a parameter storage device, and in S3, an absolute value ΔT of a temperature difference between Ta and Tb′ is obtained. Subsequently, in S4, the calculated absolute value ΔT is compared with a limit value γ, and when ΔT is larger than the limit value γ, Ta or Tb is determined to be abnormal and an alert is issued with an alarm or the like in S5, followed by issuance of a command to a correcting device in S6 not to change a correction amount set before the abnormality occurs.