To provide a method for heating fuel oil in a fuel oil storage tank to warm the fuel oil in the tank to intended temperature by flowing down heated fuel oil to the storage tank and mixing the heated fuel oil in the tank, and in particular, to partially improve warming efficiency for the fuel oil in the tank.

In this method for heating fuel oil in the fuel oil storage tank, heated fuel oil is flowed down to and mixed with the fuel oil in the fuel oil storage tank 1 via a main intake pipe 14, to warm the fuel oil in the fuel oil storage tank 1. A box shape small division 20 is formed in the storage tank 1 by surrounding the perimeter of a tip 18 of the main intake pipe 14 by a partition plate. By preventing diffusion and heat release of the heated fuel oil flowed in through the tip 18, warming efficiency for the fuel oil is partially improved.