A process for producing a carboxylic acid comprises allowing an alcohol having a carbon number of "n" to continuously react with carbon monoxide in the presence of a carbonylation catalyst system, and a limited amount of water, continuously withdrawing the reaction mixture from the reaction system 1, introducing the withdrawn reaction mixture into a distillation step (distillation columns 3a and 3b), and separating a higher-boiling component and a lower-boiling component containing a carboxylic acid having a carbon number of "n+1", respectively. In the process, the amount of carbon monoxide and/or hydrogen contained in a liquid phase of the reaction system is adjusted to at least one of the following conditions (i) and (ii): (i) the amount of carbon monoxide relative to 1 kilogram of the liquid phase by weight is at least 2 mmol per 1 MPa of carbon monoxide partial pressure of the reaction system, and (ii) the amount of hydrogen relative to 1 kilogram of the liquid phase by weight is at least 50 mmol per 1 MPa of hydrogen partial pressure of the reaction system. Such a process inhibits deactivation of a metal catalyst and deterioration in a reaction rate, and decreases formation of by-products in producing a carboxylic acid under a low water content.