To prevent sudden rise of output of a diesel engine even in the case when supply of water is suddenly stopped due to failure of a water system etc., on the diesel engine to inject fuel from a fuel system and water from the water system into a cylinder from an injection valve.

A signal to change in correspondence with water filling quantity of a fuel system 1 is sent from a controller 54 of a water system 2, when the water filling quantity is abnormally reduced, injection quantity of fuel is reduced in accordance with the reduction quantity and thereafter, the injection quantity of fuel is returned under a fuel control method of a diesel engine to inject fuel from the fuel system 1 and water from the water system 2 into a cylinder from an injection valve 20.