NOVELTY - A mixed fuel comprises acidic lignin colloid or particles and fuel oil obtained by treating paper pulp wastewater or waste liquid by acidification.    USE - Mixed fuel is used as fuel for oil tank and fuel injector of engine (claimed).    ADVANTAGE - The mixed fuel has excellent hydrophobicity, fluidity, homo-mixing property and handleability, and reduced moisture content and viscosity.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) manufacture of mixed fuel. The pH of black liquor or mixture containing alkali, acid or neutral lignin discharged by adjusting pulp waste liquid by acid hydrolysis or enzyme hydrolysis of biomass to alkali, is adjusted, such that lignin is completely dissolving in aqueous solution. The resultant aqueous solution is added with radical-free silicic acid chemical substances and silicate precipitate is removed by filtering to obtain mixture solution (a) containing small amount of inorganic impurities. The obtained mixture solution (a) is added with sulfuric acid, hydrochloric acid, nitric acid, carbonic acid or phosphoric acid to adjust the pH to 4 or less and filtering to obtain mixture (b) containing lignin. The inorganic salt particles, inorganic salt powder or saturated aqueous solution of inorganic salt is added to obtained mixture (b) to obtain mixture (c). The obtained mixture (c) is dried naturally under low temperature, dewatered slowly under low-temperature hot wind, or dewatered slowly under relative high temperature, and certain amount of humidity is maintained to obtain naturally broken or easily broken mixture powder (d). The obtained mixture powder (d) is stirred to obtain refined mixture powder (e). The obtained refined mixture powder (e) is further dried and heated to obtain mixture powder (f) containing low water content. The resultant mixture powder (f) is subjected to cyclone separation to obtain mixture powder (g) containing lignin with lower salt content. The resultant mixture powder (g) is cleaned using water or acid aqueous solution to obtain mixture (h) containing low salt content. The resultant mixture (h) is heated to obtain mixture (i) containing low water content. The resultant mixture powder (i) is stirred to obtain refined mixture powder containing lignin;    (2) engine, which has transfer pipe which is periodically cleaned using second fuel containing compound which dissolves lignin;    (3) oil tank of engine; and    (4) fuel injector of engine.