An internal combustion engine system includes an internal combustion engine, a turbocharger, and a flow ratio adjustment device including a branch configured to divide the compressed into first compressed air and second compressed air and a valve device configured to adjust a flow rate of the first compressed air and a flow rate of the second compressed air. The system additional includes a reformer configured to discharge first generated as a result of a reaction between the first compressed air and the fuel gas, a junction configured to generate second gas including the first gas and the second compressed air, an air-fuel mixture generator configured to generate an air-fuel mixture including the second gas and the fuel gas, and a controller configured to determining a ratio of the flow rate of the first compressed air based on the temperature of the air-fuel mixture.