A method for producing a fuel oil base, comprising: a first step of aerobically culturing a microalga Euglena under a nitrogen-deficient condition; a second step of adding a nutrient to a treatment solution containing the microalga Euglena cultured in the first step, adjusting the dissolved oxygen concentration of the treatment solution to 0.03 mg/L or less, and performing anaerobic fermentation of the microalga Euglena to produce a wax ester; and a third step of hydrotreating a raw material oil containing the wax ester to produce a fuel oil base.