To provide a method for estimating a concentration of carboxylic acid gas, which is capable of measuring safely the concentration of carboxylic acid gas such as formic acid gas or the like in a chamber of a soldering apparatus in real time, and a soldering apparatus capable of estimating the concentration of the carboxylic acid gas in the chamber. The method includes the steps of measuring a surface temperature of a same object placed in the chamber at a same point in time by using a thermometer (first thermometer) for measuring a temperature without any influence of infrared absorption by carboxylic acid, and a radiation thermometer (second thermometer) for measuring a temperature by infrared in a wavelength region that the carboxylic acid absorbs, and estimating the concentration of the carboxylic acid gas in the chamber on the basis of a temperature difference (ΔTx) between temperatures indicated by the first and second thermometers. The soldering apparatus includes a heating stage (30), a gas introducing part (25) for introducing a mixed gas of an inert gas and a carboxylic acid gas, a gas discharging part (26), first thermometer (37) and second thermometer (38).