A process for the preparation of a nonnatural amino acid represented by the following formula (C): which is characterized by reacting a compound represented by the following formula (A1), a compound represented by the following formula (A2) and a compound represented by the following formula (A3) (step 1): to form a compound represented by the following formula (B): and then subjecting the compound (B) to deprotection of protective groups of carboxyl groups and to decarboxylation (wherein X1 and X2 are a leaving group, R1 is an aromatic group, an unsaturated heterocyclic group, or R6CO group, R2 and R3 are a protected carboxyl group, R4 is a protected amino group, R5 is hydrogen atom, an aliphatic hydrocarbon group, an aromatic group, or a heterocyclic group, R6 is an aromatic group or an unsaturated heterocyclic group, etc., and p, q and r are an integer of 0 to 10).