A nucleic acid and a protein increasing expression levels under salt stress provide a nucleic acid with a nucleic acid sequence of SEQ. ID. NO. 1 in Sequence Listing, a variant nucleic acid thereof, a protein with an amino acid sequence of SEQ. ID. NO. 2 in Sequence Listing, and a variant protein thereof. Each of the nucleic acids and the proteins increases an expression level under salt stress. The nucleic acid and the protein are capable of providing a novel protein with a function to impart salt stress tolerance to a plant, and providing a novel gene encoding the novel protein.