PURPOSE: To measure continuously a correct atmospheric pressure by using one pressure sensor as the intake pressure detecting means for detecting the absolute pressure in a intake pipe, and obtaining the atmospheric pressure with correcting the absolute pressure of the intake air.

CONSTITUTION: A measuring apparatus has the intake pressure detecting means, e.g. a intake pressure sensor 11, detects the absolute pressure in the intake pipe 3, and outputs a crank angular signal Is3. The apparatus has a means for detecting the opening degree of a throttle valve, e.g. a throttle sensor 14, detects the opening degree of the throttle valve, and outputs a throttle valve opening degree signal Is5. The corrected value is added to measure the atmospheric pressure when the detected outputs of the throttle valve opening degree detecting means and an engine revolving number detecting means belong to the prescribed range respectively. Thereby, the correct atmospheric pressure can be measured continuously.