It is intended to provide an in vitro culture medium that allows the in vitro culture of fertilized eggs to progress normally to the blastocyst stage without arresting the development thereof. The normal development rate of a fertilized egg is improved by using 5-aminolevulinic acids represented by the following formula (I) (wherein R 1 represents a hydrogen atom or an acyl group, and R 2 represents a hydrogen atom, a linear or branched alkyl group, a cycloalkyl group, an aryl group, or an aralkyl group) or salts thereof as an agent for improving a normal development rate of a fertilized egg.