An internal combustion engine system includes: an in-cylinder pressure sensor; a crank angle sensor; and a seal portion that seals a space between an outer face of a housing and a wall surface of a cylinder head. A slope that is a ratio of the amount of decrease in a heat release amount relative to the amount of increase in a crank angle is calculated in a period during an expansion stroke from a combustion end point until an opening timing of an exhaust valve. The existence or nonexistence of an abnormality in the sealing function of the seal portion is determined based on whether or not a ratio of the amount of decrease in the slope to the amount of increase in an engine speed is greater than a threshold value.