The cleaning agent for a diesel engine includes a mixed solution obtained by blending a solvent that exhibits solubility and has an ignition point of 238° C. or more, and lubricating oil such as mineral oil in a predetermined ratio. The solvent and the grease are selected such that the cleaning agent has a higher ignition temperature characteristic than the ignition characteristic of light oil, and an evaporation characteristic that takes eight minutes or more to vaporize 2.5 cc of the cleaning agent in a heated state at 120° C.