A device of an internal combustion engine, which has a multicore processor including a plurality of cores mounted thereon and is capable of calculating various calculations relating to an operation of the internal combustion engine in parallel by the plurality of cores, includes a calculation unit that assigns a task of the calculations to at least one of the plurality of cores and performs the calculation, an EGR unit that controls an EGR operation that recirculates a gas flowing in an exhaust system of the internal combustion engine to an intake system, and a control unit that decreases a number of cores that are used in the calculation unit to be smaller as compared with before stop, when the EGR operation is stopped. The calculation unit includes an assigning unit that assigns a task of a specific calculation relating to the EGR operation to one or a plurality of designated cores, and when the EGR operation is stopped, the control unit stops the designated core or cores.