We are developing a human-riding wheeled inverted pendulum for use as a personal vehicle. To be practically usable, the vehicle requires a function that enables the rider to get on and off both safely and smoothly while on a slope of unknown angle. Concretely, for the convenience of the rider, the vehicle needs to be stabilized on an unknown slope without the rider aboard, both before the rider gets on it and after he/she gets off it. Moreover, the vehicle should be stabilized safely relative to the rider's handling force at a grip of the vehicle while getting on and off. We thus developed a method of estimating the handling force and the slope angle separately. In this paper, we report the method to estimate the handling force and the slope angle using a disturbance observer. We verified the validity of our proposed estimation method by computer simulations and experiments using a prototype of a human-riding wheeled inverted pendulum on an unknown slope.