An adaptive controller using an adjustment law expressed in recurrence formula, including a plant, a controller which controls an input to the plant, and a parameter adjuster which adjusts a parameter of the controller. In the controller, an input to the parameter adjuster is multiplied by a coefficient other than 1 such that the range of change of the parameter adjuster's internal variable is restricted to a predetermined limit. With this arrangement, the controller can be realized on a less expensive and fewer-bit-length microcomputer such as 16-bits. The input to the parameter adjuster may be made up of the control input to the plant and an output therefrom. Alternatively, the input to the controller and an output from the plant may be both multiplied by a coefficient other than 1 to be input to the parameter adjuster. Further, it may be arranged such that a gain matrix element of the parameter adjuster is intermittently replaced with a prescribed value when it exceeds a limit, or when the cycle of the control exceeds a predetermined number of cycles.