To determine on line appropriate replacement timing for differential pressure/pressure composite sensors.

A secondary side pressure sensor 5 is provided separately from a differential pressure/pressure composite sensor 4. Every time a certain period passes, a differential pressure signal ΔP from the differential pressure sensor chip 43, a static pressure signal PG from a pressure sensor chip 46 and a secondary side pressure signal P2 from the secondary side pressure sensor 5 are acquired, and an evaluation indicator P0 is figured out by the following equation: P0=|ΔP - (PG - P2)|. The replacement timing of the differential pressure/pressure composite sensor 4 is determined from the trend of this evaluation indicator P0.