(1) Primary endpoint Comparison of asthma exacerbation rates of omalizumab-treated vs non-Omalizumab treated group at outcome period (6 months). (asthma exacerbation: at least 1 systemic steroid treatment and/or bursts of systemic oral steroid use by prednisone-equivalent dose ≥45mg/3days, in a 6 month for relieving asthma exacerbations.) Fisher's exact test is examined for asthma exacerbation between omalizumab-treated vs non-Omalizumab treated group.