PURPOSE: To always enable an accurate abnormality discrimination eliminating misconception by discriminating abnormalities in respective sensors based on both output values of two sensors having output characteristics different from each other with changes in a specified physical quantity.

CONSTITUTION: Two pressure sensors provided at the downstream of a throttle valve in an engine suction pipe are for the low range (PBNA) and the high range (PBTC) and have different output voltage characteristics. For example, a discrimination is made in the magnitude between the output value VPBTC and the second set value VPBTC17 of the high-range sensor and when VPBTC<VPBTC17, a discrimination is made between the output value VPBNA and the first set value VPBNAH. When VPBNA>VPBNAH, it is determined whether the state continues for a fixed period or not. When affirmative, the low range sensor is judged to be abnormal and an alarm of the abnormality thereof is given. In this manner, the abnormality of the high range sensor is also judged based on the output of both the sensors.