An object of the invention is to realize high-accuracy analysis of a flue gas in terms of a trace amount of ammonia so as to control the level of ammonia gas to be added to a flue gas NOx removal apparatus. Since the ammonia gas to be determined is a highly absorbable, the gas component is considerably adsorbed on a surface of the flue gas sampling tube during sampling of flue gas. When a conventional sampling method including heating a gas sampling tube is employed, the amount of sampled gas is reduced through adsorption during gas sampling, failing to obtain concentration values at high accuracy. Thus, a high-accuracy gas sampling method replacing a conventional, heating-based method includes completely washing out, with an absorption liquid therefor serving as a washing liquid, a target ammonia gas deposited on the inner wall of a flue gas sampling tube (2), whereby the gas component is thoroughly collected. Through analysis of the thus-obtained washing liquid, high-accuracy analysis can be attained.