To provide an attitude-angle detecting apparatus, which detects an attitude angle of a mobile object during movement with good accuracy by correcting an output value from an acceleration sensor, and to provide a method for the same. It is characterized in that it comprises an acceleration sensor for measuring an acceleration being applied to a mobile object, a yaw-rate sensor for measuring a yaw rate of the mobile object, a speed sensor for measuring a speed of the mobile object, a mobile-component acceleration calculating means for calculating an actual acceleration from the speed, calculating a centrifugal force from the yaw rate and the speed and calculating a mobile-component acceleration, a resultant force of the actual speed and the centrifugal force, and an attitude-angle calculating means for calculating an attitude angle from a gravitational acceleration, which is obtainable by correcting the acceleration with the mobile-component acceleration.