Fuel level detecting device for fuel tanks

To provide a fuel level detecting device for a fuel tank to which its fuel level detecting unit is capable of being mounted in a position so that its maximum allowed fuel level is easily detected even in the case of variously shaped fuel tanks, a fuel level detecting device for a fuel tank (20) having a fuel level detecting unit (24) disposed to be adapted to support a float capable of oscillating (26) in the fuel tank (20) with a built-in fuel pump (19) and detecting the fuel level through the oscillations of the float (26); wherein the fuel level detecting unit (24) is capable of being selectably mounted to the fuel pump (19) such that the float (26) oscillates at the different positions in the fuel tank (20).