A tire angular velocity controller (211) is input with a difference between a target value of a rotational angular velocity of 0 for main wheels (11) and a rotational angular velocity of the main wheels (11), which is a differential value of a signal output from a main wheels rotary encoder (26). The tire angular velocity controller (211) calculates an inclination angle for the main body (10) that will cause the difference to become zero. In a second control mode, the calculated inclination angle is used as a target inclination angle and the difference between this target inclination angle and the inclination angle of the main body (10) at the present time input from an inclination angle sensor (20) is input to a main body inclination angle controller (212).