To reduce the generation of NOx and improve fuel economy as a whole during a transient period in which an air excess factor and oxygen concentration reach respective target values.

A control device for a diesel engine comprises an air excess factor regulating means 71 to enable regulation of an air excess factor of gas sucked in an engine; an oxygen concentration regulating means 72 capable of regulating oxygen concentration of gas sucked in the engine; means 73 and 74 to set an air excess factor and oxygen concentration according to an operation condition; and a variation operation regulating means 75 to regulate a route of a change of oxygen concentration to an air excess factor during variation operation to the side, where oxygen concentration is low, by regulating a time required for at least one variation operation of an air excess by the air excess regulating means and variation operation for oxygen concentration by the oxygen concentration regulating means or a starting timing of variation operation when a target value of oxygen concentration is changed to the high side due to a change of an operation condition.