A deposit accumulation amount estimation device estimating an inlet deposit accumulation amount and an outlet deposit accumulation amount by calculating those amounts, the inlet deposit accumulation amount being an amount of combustion product accumulating on an injection hole inlet area, the outlet deposit accumulation amount being an amount of combustion product accumulating on an injection hole outlet area, characterized in that a combustion product production amount estimation device configured to estimate an inlet combustion product production amount and an outlet combustion product production amount by calculating those amounts, the inlet combustion product production amount being an amount of combustion product produced due to the combustion of fuel at the injection hole inlet area constituted by: an area inside of the fuel injection hole of the fuel injector and at an inlet side of the fuel injection hole; and an area outside of the fuel injection hole and adjacent to an inlet of the fuel injection hole, the outlet combustion product production amount being an amount of combustion product produced due to the combustion of fuel at an injection hole outlet area constituted by: an area inside of the fuel injection hole and at an outlet side of the fuel injection hole; and an area outside of the fuel injection hole and adjacent to an outlet of the fuel injection hole, the inlet combustion product production amount being calculated on the basis of a temperature of the injection hole inlet area, the outlet combustion product production amount being calculated on the basis of a temperature of the injection hole outlet area, the temperature of the injection hole inlet area and the temperature of the injection hole outlet area being obtained separately, and in that a deposit remove amount estimation device configured to estimate an inlet deposit remove amount and an outlet deposit remove amount by calculating those amounts, the inlet deposit remove amount being an amount of removed combustion product among combustion product accumulating on the injection hole inlet area constituted by: an area inside of the fuel injection hole of the fuel injector and at an inlet side of the fuel injection hole; and an area outside of the fuel injection hole and adjacent to the inlet of the fuel injection hole, the outlet deposit remove amount being an amount of removed combustion product among combustion product accumulating on an injection hole outlet area constituted by: an area inside of the fuel injection hole and at the outlet side of the fuel injection hole; and an area outside of the fuel injection hole and adjacent to the outlet of the fuel injection hole, the inlet deposit remove amount being calculated on the basis of an inlet removal coefficient KR in determined as a function of an inlet deposit accumulation amount defined as an amount of combustion product accumulating on the injection hole inlet area, the outlet deposit remove amount being calculated on the basis of an outlet removal coefficient KR out determined as a function of an outlet deposit accumulation amount defined as an amount of combustion product accumulating on the injection hole outlet area, the inlet deposit accumulation amount being calculated by subtracting the inlet deposit remove amount calculated by the deposit remove amount estimation device from the inlet combustion product production amount calculated by the combustion product production amount estimation device, and the outlet deposit accumulation amount being calculated by subtracting the outlet deposit remove amount calculated by the deposit remove amount estimation device from the outlet combustion product production amount calculated by the combustion product production amount estimation device.