To provide a new thermostable protein which has mevalonate kinase activity whose optimal pH is acidic.

This protein comprises the following amino acid sequence (1) or (2). (1) A specific amino acid sequence having the enzymatic activity and originated from a hyper-thermophilic old archaebacterium, Aeropyrum pernix. (2) An amino acid sequence which is (1) the amino acid sequence whose one or more amino acid residues are deleted, added or replaced, and which provides a thermostable protein having at 45C or higher mevalonate kinase activity whose optimal pH is acidic.