An integrated coal gasification combined cycle system comprising a calorific abnormality detection device that defines, based on a relation between the amount of at least one of char, air, coal, and oxygen charged to a coal gasification furnace and a power generation output of a gas turbine, an allowable variation range of the power generation output of the gas turbine with respect to the charged amount or an allowable variation range of the charged amount with respect to the power generation output of the gas turbine, and that detects a calorific abnormality when an actually charged amount or an actual power generation output of the gas turbine falls outside the allowable variation range.