Controlling excess air ratio is a challenging issue in maximizing energy efficiency of a reheating furnace. This paper proposes a new methodology for controlling the excess air ratio using a Tunable Diode Laser Absorption Spectroscopy (TDLAS) sensor while most industries have been using Zr sensor. An oxygen concentration feedback control system using the methodology is also developed and implemented in real reheating furnaces in South Korea. The proposed TDLAS sensor control system turned out to be better than a Zr based one in terms of maintaining the desired performance steadily throughout the entire heating region of the reheating furnace.