The ferrite system heat-resistant cast steel and the exhaust system component are provided, which are inexpensive and are able to improve the reliability by largely improving the toughness under normal temperature and thermal fatigue performance. The ferrite system heat-resistant cast steel includes composition structure comprised, percent by mass, of 0.1% to 0.4% carbon, 0.5% to 2.0% silicon, 0.2% to 1.2% manganese, 0.3% or less phosphorus, 0.01% to 0.4% sulfur, 14.0% to 21.0% chrome, 0.05% to 0.6% niobium, 0.01% to 0.8% aluminum, 0.15% to 2.3% nickel, residual iron and inevitable impurities.