The present invention provides a new separation functional layer suitable for separating an acid gas from a gas mixture containing the acid gas. A separation functional layer 1 of the present invention includes polyimide P. The polyimide P includes a structural unit A1 derived from a tetracarboxylic dianhydride a1 having a six-membered ring acid anhydride structure, and a structural unit B1 derived from diamine b1. At least one of the structural unit A1 and the structural unit B1 has at least one functional group F selected from the group consisting of a carboxyl group, a hydroxyl group, a thiol group, and metal salts of these groups.