PURPOSE: To secure always detection precision of almost zero differential pressure of a differential pressure sensor with high accuracy by performing zero- point calibration without any limitation of frequency on such an engine condition that a negative pressure value is not perpendicular.

CONSTITUTION: The differential pressure sensor 10 is provided on the upstream and downstream sides of the throttle valve 11 of an intake pipe through communication paths 100, 101, and 102 to detect the differential pressure between the upstream and downstream sides of the throttle valve 11. A short-circuit path 103 is provided between the upstream-side communication path 100 and downstream- side communication path 101. The short-circuit path 103 is driven by a solenoid valve 20 provided between the communication paths 101 and 102. The solenoid valve 20 is energized on such an engine condition that the negative pressure value is not important, e.g. when an ignition switch is turned on or off.