Disclosed herein is a fiber capable of basic gas absorption and easy regeneration. Disclosed also herein is a process for producing said fiber. The basic gas absorptive fiber is an acrylic fiber characterized by a specific amount of nitrogen which is increased by crosslinking with hydrazine, a specific amount of carboxyl groups and amido groups resulting from modification of nitrile groups, and a specific value of tensile strength. It is prepared from acrylic fiber by crosslinking with hydrazine and subsequent hydrolysis and conversion of hydroxyl groups into carboxylic acid. It has good processability and can be used repeatedly.