An internal cleaning agent for a diesel engine to be sprayed from an intake system with a diesel engine running to clean and remove carbon and sludge deposits on the inside of the engine, the cleaning agent comprising a mixed solution in which a solvent and a lubricating oil are blended, wherein the solvent has an ignition temperature of 238°C or more, and wherein said solvent has ability of weakening formability of carbon deposit layer by attaching to and penetrating the carbon deposit layer, and exertion of sludge-dissolving power, the lubricating oil comprises any of mineral oil, paraffin oil, naphthenic oil, poly-α-olefin (PAO: poly-α-olefin), vegetable oil, or a combination thereof, and has a property as engine oil to be used to improve adhesion and a deposition time of the solvent, the mixed solution is formed by blending the lubricating oil and the solvent, and the lubricating oil and the solvent are selected in such a manner that the mixed solution has the following properties as well as evaporation characteristics: the properties include having an ignition temperature higher than 240 °C, and preventing a knocking phenomenon before a fuel injection timing and at the end of the compression stroke from a relationship between the ignition timing and a flame propagation speed, and the evaporation characteristics include an evaporation time of eight minutes or more for vaporizing 2.5 cc of the mixed solution in a heated state of an aluminum plate at 120°C, wherein the solvent is ethylene glycol monobutyl ether and a blending ratio of the solvent to the lubricating oil is within a range of 95: 5 to 80: 20 in the weight ratio, or wherein the solvent is ethylene glycol mono-tertiary butyl ether, and a blending ratio of the solvent to the lubricating oil is within a range of 99: 1 to 80: 20 in the weight ratio. the solvent has an ignition temperature of 238°C or more, and wherein said solvent has ability of weakening formability of carbon deposit layer by attaching to and penetrating the carbon deposit layer, and exertion of sludge-dissolving power, and wherein said solvent has ability of weakening formability of carbon deposit layer by attaching to and penetrating the carbon deposit layer, and exertion of sludge-dissolving power, the lubricating oil comprises any of mineral oil, paraffin oil, naphthenic oil, poly-α-olefin (PAO: poly-α-olefin), vegetable oil, or a combination thereof, and has a property as engine oil to be used to improve adhesion and a deposition time of the solvent, the mixed solution is formed by blending the lubricating oil and the solvent, and the lubricating oil and the solvent are selected in such a manner that the mixed solution has the following properties as well as evaporation characteristics: the properties include having an ignition temperature higher than 240 °C, and preventing a knocking phenomenon before a fuel injection timing and at the end of the compression stroke from a relationship between the ignition timing and a flame propagation speed, and the evaporation characteristics include an evaporation time of eight minutes or more for vaporizing 2.5 cc of the mixed solution in a heated state of an aluminum plate at 120°C, wherein the solvent is ethylene glycol monobutyl ether and a blending ratio of the solvent to the lubricating oil is within a range of 95: 5 to 80: 20 in the weight ratio, or wherein the solvent is ethylene glycol mono-tertiary butyl ether, and a blending ratio of the solvent to the lubricating oil is within a range of 99: 1 to 80: 20 in the weight ratio. the properties include having an ignition temperature higher than 240 °C, and preventing a knocking phenomenon before a fuel injection timing and at the end of the compression stroke from a relationship between the ignition timing and a flame propagation speed, and the evaporation characteristics include an evaporation time of eight minutes or more for vaporizing 2.5 cc of the mixed solution in a heated state of an aluminum plate at 120°C, wherein the solvent is ethylene glycol monobutyl ether and a blending ratio of the solvent to the lubricating oil is within a range of 95: 5 to 80: 20 in the weight ratio, or wherein the solvent is ethylene glycol mono-tertiary butyl ether, and a blending ratio of the solvent to the lubricating oil is within a range of 99: 1 to 80: 20 in the weight ratio.