To obtain a high output power on the 251-μm line of a CH3OH laser, four types of far-infrared output mirror are examined. It is found that the capacitive aluminium-mesh mirror of the Danielewicz type, which is fabricated by conventional vacuum deposition techniques in the present work, selectively extracts an output power of 18 mW from a 25-W pump. Also, exact assignment of the line is achieved by tuning the far-infrared cavity using the mirror.