A fuel for a diesel engine is produced by using a fish waste oil or a mixture of a vegetable waste oil and the fish waste oil which have conventionally been wasted as a staring material. Fish oil (virgin oil or fish waste oil) or a mixture of the fish oil and vegetable oil (virgin oil or vegetable waste oil) (hereinafter referred to as starting material) is subjected to stirring treatment while introducing ozone, and an oxidation-reduction agent and a polymerization inhibitor are added during the stirring treatment. Moreover, it comprises the respective steps of a first treatment step in which a stirring treatement is carried out while introducing ozone to the above-mentioned starting material, and an oxidation-reduction agent and a polymerization inhibitor are added during the stirring treatement, a second treatment step in which the material obtained in the above-mentioned first step is filtered and a stirring treatment is further carried out while introducing ozone again, and a step of introducing a vegetable additive to the resulting material by the second treatment step.