A mobile body including a vehicle body having a boarding section (4, 204, 404, 504) for an operator and freely tiltable in a roll direction with respect to a road surface, front and rear wheels disposed spaced apart from each other in a longitudinal direction of the vehicle body, an actuator (15, 37, 209) capable of causing a moment in the roll direction to act on the vehicle body, and a control device configured to control the actuator, the control device comprising: a center-of-gravity displacement degree index value determining section (83, 111, 283, 301) which determines a center-of-gravity displacement degree index value (Pb_err, Pb_err1, φb_err, φb_err1) using an observed value (Pb_diff_y_act, φb_act) of a motional state of the mobile body, the center-of-gravity displacement degree index value representing an estimate of a degree of displacement of a center of gravity of the operator boarded on the boarding section in a lateral direction of the vehicle body from a predetermined reference position with respect to the vehicle body; and a control input determining section (85, 102, 113, 120, 285, 303) which determines a control input (δf1_dot2_cmd, Msum_cmd) for controlling the actuator (15, 37, 209) in accordance with the determined center-of-gravity displacement degree index value, wherein the control device is configured to control the actuator in accordance with a determined control input; wherein the control input determining section (85, 102, 113, 120, 285, 303) determines the control input (δf1_dot2_cmd, Msum_cmd) for controlling the actuator (15, 37, 209) in accordance with the determined center-of-gravity displacement degree index value in such a way as to increase the degree of displacement of the center of gravity of the operator indicated by the determined center-of-gravity displacement degree index value (Pb_err, Pb_err1, φb_err, φb_err1) in a state where the center of gravity of the operator boarded on the boarding section is displaced from the predetermined reference position, characterized in that the actuator is an actuator which steers a steered wheel, among the front and rear wheels, so as to cause a ground contact point of the steered wheel to move laterally. a center-of-gravity displacement degree index value determining section (83, 111, 283, 301) which determines a center-of-gravity displacement degree index value (Pb_err, Pb_err1, φb_err, φb_err1) using an observed value (Pb_diff_y_act, φb_act) of a motional state of the mobile body, the center-of-gravity displacement degree index value representing an estimate of a degree of displacement of a center of gravity of the operator boarded on the boarding section in a lateral direction of the vehicle body from a predetermined reference position with respect to the vehicle body; and a control input determining section (85, 102, 113, 120, 285, 303) which determines a control input (δf1_dot2_cmd, Msum_cmd) for controlling the actuator (15, 37, 209) in accordance with the determined center-of-gravity displacement degree index value, wherein the control device is configured to control the actuator in accordance with a determined control input; wherein the control input determining section (85, 102, 113, 120, 285, 303) determines the control input (δf1_dot2_cmd, Msum_cmd) for controlling the actuator (15, 37, 209) in accordance with the determined center-of-gravity displacement degree index value in such a way as to increase the degree of displacement of the center of gravity of the operator indicated by the determined center-of-gravity displacement degree index value (Pb_err, Pb_err1, φb_err, φb_err1) in a state where the center of gravity of the operator boarded on the boarding section is displaced from the predetermined reference position, characterized in that the actuator is an actuator which steers a steered wheel, among the front and rear wheels, so as to cause a ground contact point of the steered wheel to move laterally.