A method of continuously producing polyalkylbiphenyls involves reacting biphenyl and an olefin in the presence of a solid acid catalyst to obtain a reaction mixture containing monoalkylbiphenyls and dialkylbiphenyls, separating a fraction containing biphenyl and at least a part of the monoalkylbiphenyls, circulating the separated fraction to the reactor such that the ratio of biphenyls to monoalkylbiphenyls is designed to be 0.1 or more and is designed to be less than the soluability of biphenyl to monoalkylbiphenyl at a circulation temperature, and recovering polyalkylbiphenyls.