The present specification discloses a method for detecting an aneuploid of a Brassica oleracea plant, including: performing real-time PCR using DNA extracted from a sample derived from a Brassica oleracea plant to be tested as a template and DNA markers specific to each of two or more chromosomes of Brassica oleracea plant; and detecting chromosomal aneuploidy from a relative difference in amplification amount between the obtained DNA markers. According to one embodiment of the present invention, it is possible to simply, accurately, and rapidly detect an off-type (chromosomal aneuploid) that may occur in Brassica oleracea varieties, in a laboratory equipped with a general molecular biological equipment in the course of seed quality control and breeding research.