1. A control device for a mobile body which controls a mobile body having a base body and a moving mechanism, which moves the base body on a floor surface, such that the mobile body travels while moving the base body up and down, the control device comprising: up-and-down-direction inertial force parameter determining device which determines an up-and-down-direction inertial force parameter, which is a parameter that defines the time series of a desired inertial force in the up-and-down direction of the mobile body or the base body; desired motion determining device which determines the time series of a desired motion of the mobile body by using at least the up-and-down-direction inertial force parameter such that the time series of the desired inertial force defined by the determined up-and-down-direction inertial force parameter is satisfied; operation control device which controls the operation of the mobile body on the basis of at least the determined desired motion; state amount deviation observing device which sequentially observes a state amount deviation indicating the degree of deviation of an actual posture of the base body from a desired posture of the base body in the determined desired motion; and required manipulated variable determining device which sequentially determines a required manipulated variable, which defines an additional external force to be additionally applied to the mobile body so as to converge the state amount deviation to zero, according to a feedback control law on the basis of an observed value of the state amount deviation, wherein the operation control device is a device which controls the operation of the mobile body so as to additionally apply the additional external force defined by the required manipulated variable to the mobile body while causing an actual motion of the mobile body to follow the desired motion, and the required manipulated variable determining device includes feedback gain determining device which determines a feedback gain of the feedback control law necessary for converging the state amount deviation in the future to zero at each time, at which the required manipulated variable is determined, by using the time series in a period from current time to predetermined time in the future in the time series of a desired inertial force defined by the up-and-down-direction inertial force parameter, and determines the required manipulated variable by computation according to the feedback control law from the determined feedback gain and an observed value of the state amount deviation.