A current increase-decrease amount (ΔIγ*) computed by a command current increase-decrease amount computing unit is added to an immediately preceding value (Iγ*(n−1)) of a command current value (Iγ*) in an adder. The command current value (Iγ*) obtained by the adder is given to a high/low limit limiter. The high/low limit limiter limits the command current value (Iγ*), obtained by the adder, to a value between a low limit value (ξmin (ξmin≧0)) and a high limit value (ξmax (ξmax>ξmin)). A high limit value setting unit obtains the high limit value (ξmax) corresponding to the vehicle speed detected by the vehicle speed sensor, from a vehicle speed-vs.-high limit value map set by a map creating/updating unit, and sets the obtained high limit value (ξmax) in the high/low limit limiter.