To enhance detection accuracy of a gradient of a traveling road by correcting the zero point of a detected acceleration (G sensor acceleration) of an acceleration sensor, while also considering an influence of a change in the posture of its vehicle caused by loading/unloading of its passenger or an object.

Zero point correction with respect to the G sensor acceleration is performed by a gradient detection section 9 according to the posture of the vehicle which changes due to the loading/unloading of the passenger or the object, based on a change in the G sensor acceleration during a vehicle posture learning period from the detection of opening of a door of the vehicle 1 up to the detection of its closing, when the opening and closing of the door performed for loading/unloading the passenger or the object are detected with a courtesy switch 5. When constant-speed travel on a flat road is detected on the basis of the G sensor acceleration and an acceleration (actual acceleration) calculated from a change in vehicle speed detected by a vehicle speed sensor 6, zero point correction with respect to the G sensor acceleration is further performed according to the difference between the actual acceleration during the constant-speed travel on the flat road and the G sensor acceleration, by the gradient detection section 9.