Provided are a device for producing fuel and a method for producing fuel that: remove ammonia, which inhibits methane fermentation of animal manure; refine organic matter such as carbohydrates, proteins, and fats included in manure; and lower the molecular weight of said organic matter, thereby improving the methane fermentation rate and increasing the amount of generated gas. The method for producing a fuel is characterized by including: a step for introducing animal manure into a reaction vessel and injecting, into the reaction vessel, water corresponding to 5 to 20 times (inclusive) the amount of animal manure introduced; a step for injecting 0.62 to 1.9 MPa of saturated water vapor into the reaction vessel after the animal manure and the water have been introduced; a step for starting stirring inside the reaction vessel while the water vapor continues to be injected, removing ammonia, and separating bonding molecules in the organic matter included in the animal manure to refine the organic matter; and a s
