To provide a control device of an internal combustion engine, accurately estimating presumed temperature of exhaust system equipment of the internal combustion engine in a simple way.

The control device 30 includes: a stopping duration determination means 31 determining whether operation stopping duration of an internal combustion engine body 12 is within a predetermined duration range; a presumed temperature calculation means 32 calculating the presumed temperature of the exhaust system equipment (an air-fuel ratio sensor 18 and a catalyst device 19) based on cooling water temperature, etc. during steady operation of the internal combustion engine body 12 and setting the presumed temperature calculated at the end of previous operation as a calculated value when the operation stopping duration of the internal combustion engine body 12 is within the predetermined duration range at starting; a calculated value storing means 33; an averaging number calculation means 40 as a means applying averaging to the calculated value of the presumed temperature; an operation duration determination means 41; an averaging number correction means 42; and an averaging means 43.