To provide a holder of a magnetostrictive liquid level gauge detection unit for allowing a calibrating/water checking operation by utilizing a mounting tube, without having to remove the detection unit from the mounting tube.

The holder 10 has a flange 11 placed on the upper end face of the mounting tube 20. The flange 11 has a guide pipe slot 11a for passing and holding a guide pipe 31 of the magnetostrictive liquid level gauge detecting unit 30 and a measuring rod slot 11b for passing a measurement rod 40, which is used to measure the liquid level of a liquid storage tank. The guide pipe slot 11a is provided vertically, at a position which is biased from the center of the flange 11 in a first direction, and the measuring rod slot 11b is provided in a position, biased from the center of the flange 11 in a second direction opposite the first direction, in such a way that the measurement rod slot is tilted toward the center of the flange 11.