This paper discusses a nonlinear output regulation problem for linear systems. A system is often expressed by the coordinate with physical meaning, for example, position, angle, and velocity etc. However a coordinate after a nonlinear transformation has few physical meaning. Thus an output of exactly linearized systems is generally nonlinear function. This paper proposes a method to establish a regulation for linear systems with nonlinear output. The proposed method is applied to the acrobot system, and the validity of the method is verified via numerical simulations.