PURPOSE: To diagnose the presence or absence of abnormality of a sensor part despite a hysteresis recognized in the relation between the valve voltage and a flow rate by deciding the presence or absence of the abnormality in a prescribed working cycle.
CONSTITUTION: A delay measuring part 14 measures the valve driving voltage V corresponding to the mass flow rate setting signal (g) of each step in a prescribed working cycle and with a proper stable time secured for each step. A reference valve driving voltage storing part 15 stores the voltage V as the reference valve driving voltage V0n. The actual valve driving voltage Vn set based on a sensor signal S is measured via a comparison control part 21 after the same stabilizing time and in a working mode. The measured voltage Vn is supplied to a comparison deciding part 16. Then, the part 16 compares the allowance value received from an allowance value setting part 17 and added to the voltage Vn with the voltage V0n. When the result of this comparison exceeds a prescribed range and also exceeds a prescribed time set by a timer 18, the part 16 displays a fault through an alarm means 19. Thus, the fault diagnosing accuracy is improved.