Ammonia engine system

An ammonia engine system includes a combustor configured to generate combustion gas by burning ammonia mixed with air, a reforming catalyst configured to be warmed up by the combustion gas, an ammonia engine configured to be supplied with hydrogen that is discharged from the reforming catalyst, and a controller. The controller is configured to, during startup of the ammonia engine, execute a combustion process that causes the combustor to generate the combustion gas and a supplying process that supplies the reforming catalyst with ammonia together with air. The controller is configured to initiate the supplying process after the combustion process has begun.