The present invention describes a method for generating a serine derivative and an optically active isomer thereof by a convenient technique, and an enzyme and the like useful in the method. In the presence of the following protein (A) and/or (B) having an enzymatic activity, an α-amino acid is reacted with an aldehyde to form a serine derivative: (A) a protein comprising the amino acid sequence of SEQ ID NO:5, and (B) a protein comprising an amino acid sequence of SEQ ID NO: 5, but which includes substitution, deletion, insertion and addition of one or more amino acids and is able to catalyze the reaction to form the serine derivative.