Male sterility (MS) has been studied intensively in Brassica crops to ensure high purity and the protection of $F_1$ hybrids. In the preceding trial, broccoli microspores were treated with a mutagen, n-nitroso-n-methyl urethane (NMU), during the cultural process to produce MS. Many MS plants were derived from the 0.01 ${mu}M$ and 0.1 ${mu}M$ treatments just before and at the fourth day of culture, respectively. In this study, most MS plants inherited sterility from a single recessive gene. Only one plant, 9-4C-106, which originated from the 0.1 ${mu}M$ treatment on the fourth day, was controlled by a single dominant gene. Another dominant mutant gene suppressing the expression of this dominant MS gene was discovered in a plant treated with 0.01 ${mu}M$ NMU. A hybrid breeding system using this MS structure is discussed.