We investigate a controller order reduction method using singular perturbation approximation (SPA) of balanced systems. SPA of balanced systems is proposed to reduce the controller orders for a class of systems and controllers. The resulting reduced-order controllers, if they exist, are guaranteed to stabilize the corresponding systems. Furthermore, we show that SPA of closed-loop balanced systems has the same error bound as the structurally balanced truncation method of Zhou, D’Souza and Cloutier, but has zero steady-state error.