A fuel injection device is used in an internal combustion engine having a combustion chamber partitioned by a cylinder head, a cylinder, and a piston crown surface so that at least one of the amount of NOx, Pmax, and a thermal efficiency η is maintained at a predetermined value. The fuel injection device includes a fuel injection change unit. The fuel injection change unit virtually divides the combustion chamber into N number of combustion zones where N is a natural number of 2 or more, and can change a fuel injection method according to the respective combustion zones. The fuel injection change unit divides the combustion chamber into the N number of combustion zones, thereby being capable of eliminating a difference of heat in the respective combustion zones, and precisely controlling an in-cylinder pressure P in the combustion chamber. As a result, the amount of NOx and the thermal efficiency can be optimized. Therefore, both of a reduction in the amount of NOx and the high thermal efficiency can be achieved.