PURPOSE: To permit to mix the lubricating oil, whose amount is determined in accordance with the same of supplied fuel, into the fuel tank by a method wherein a lubricating oil metering tank, metering the lubricating oil, whose amount is in accordance with the amount of the supplied fuel, is provided in the fuel tank.

CONSTITUTION: An expandable and contractable lubricating oil metering tank 5 is inserted into the upper part of a fuel tank 1 from the outside of the fuel tank. The lubricating oil metering tank 5 is provided with an opening 9 and an opening and closing valve 7 is formed by covering the opening 8 by a valve body 9 capable of opening and closing the opening 8 while the interior thereof is communicated with the interior of the fuel tank 1 through the opening and closing valve 7. Upon supplying the fuel, when the tank is filled with the fuel and a float 14 is floated, an end part 5a is pushed and the volume of the lubricating oil metering tank 5 is decreased. Accordingly, the lubricating oil, filling the lubricating oil metering tank, opens the opening and closing valve 7 and the lubricating oil, whose amount is determined in accordance with the amount of the fuel supplied thereinto, is supplied into the fuel tank 1.