According to a vehicle 1 of the present invention, coordinate systems of a past estimated value and an instantaneous provisional value of a two-dimensional floor inclination are conformed, by coordinate converting at least one of the “past estimated value” and the “instantaneous provisional value” of the two-dimensional floor inclination, using an orientation of the vehicle 1 or a rate of change thereof. Thereafter, a new estimated value of the two-dimensional floor inclination is calculated, by calculating a weighed mean value or a low-pass filter value using the past estimated value and the instantaneous provisional value of the two-dimensional floor inclination.