To provide a natural gas reforming engine system having excellent thermal efficiency and capability of supplying natural gas to an engine by reforming it, by using exhaust gas, which has been recirculated to the engine as EGR gas in the past, as an additive gas for reforming crude gas such as natural gas and the like, without using a carbon dioxide separating device.

In the natural gas reforming engine system having the engine, a gas reforming device, and a heat exchanger for heating crude gas, the heat exchanger for heating the crude gas and the gas reforming device are provided in a fuel supply passage to link a crude gas supply source to the engine in this order from its upstream, the gas reforming device is connected to the exhaust passage of the engine to use the heat of the exhaust gas of the engine as a heat source, and the exhaust branch passage of the exhaust passage is connected to the fuel supply passage in the upstream of the gas reforming device to mix a part of the exhaust gas in the crude gas.