A method for producing a base oil for lubricant oils comprising: a first step of hydrocracking a stock oil having a content percentage of a heavy matter of 80% by mass or more so that a crack per mass of the heavy matter is 20 to 85% by mass, to obtain a hydrocracked oil comprising the heavy matter and a hydrocracked product thereof, a second step of fractionating the hydrocracked oil into a base oil fraction comprising the hydrocracked product and a heavy fraction comprising the heavy matter and being heavier than the base oil fraction, respectively, a third step of isomerization dewaxing the base oil fraction from the fractionation in the second step to obtain a dewaxed oil, wherein the heavy fraction from the fractionation in the second step is returned to the first step as a part of the stock oil.