The present invention relates to a method for determining copy-number variation in a mixture of nucleic acids which are known or considered to be different in the amount of one or more target sequences, and more particularly, to a method for determining copy-number variation including a bioinformatic analysis method and a statistical analysis method for interpreting variability occurring between chromosomes and between sequencings. A variation determination according to the present invention can be used to determine chromosomal copy-number variation which is associated with or considered to be associated with a medical condition of a fetus. The chromosomal copy-number variation that can be determined according to the method of the present invention may comprise a deletion and/or duplication of the trisome and monosome of any one or more from among chromosomes 1-22, X and Y, the polysome for the entire nucleic acid sequence, and any one or more sequence fragments in the chromosomes, and therefore, is useful fo
