A method for preparing a fumarate salt of a compound represented by Formula (II): (wherein R1 represents a hydrogen atom or a lower alkyl group, and R2 represents a lower alkyl group), which comprises carbamating a compound represented by Formula (I): (wherein R1 represents a hydrogen atom or a lower alkyl group), removing all carbamate groups from this compound, alkylating the nitrogen atom at the 3′-position of the desosamine ring in the resulting compound to give the compound represented by Formula (II), and converting this compound into a fumarate salt, wherein the compound represented by Formula (I) is carbamated in the presence of a cyclic ether or a carboxylic ester. This method enables efficient preparation of high-quality erythromycin derivatives.