An internal combustion engine system basically has an internal combustion engine, a reforming fuel injection device and a reformer. The internal combustion engine has an exhaust circulation path that is connected to an exhaust path and an intake path communicating with a combustion chamber. The reforming fuel injection device injects a reforming fuel into an exhaust gas flowing through the exhaust circulation path. The reformer has a reforming catalyst for generating a hydrogen-containing gas using the reforming fuel. The internal combustion engine system has a reforming catalyst regeneration device for causing an oxygen-containing gas to flow through the exhaust circulation path and thereby regenerate the reforming catalyst at a predetermined timing for regenerating the reforming catalyst.