The present invention relates to a process for synthesizing or amplifying efficiently a nucleic acid comprising a target nucleic acid sequence. In the process according to the present invention, a primer comprising in its 3′-end portion a sequence (Ac′) which hybridizes a sequence (A) in the 3′-end portion of the target nucleic acid sequence, and in the 5′-side of said sequence (Ac′) a sequence (B′) which hybridizes the complementary sequence (Bc) of a sequence (B) positioned in the 5′-side of said sequence (A) on the target nucleic acid sequence, wherein {X−(Y−Y′)}/X is in the range of −1.00 to 1.00, in which X denotes the number of bases in said sequence (Ac′), Y denotes the number of bases in the region flanked by said sequences (A) and (B) in the target nucleic acid sequence, and Y′ denotes the number of bases in an intervening sequence between said sequences (Ac′) and (B′) (Y′ may be zero).