An object of the present invention is to provide a mounting member structure capable of preventing unstable vibration of an engine mount. A mounting member structure, which is mounted on a power plant including an engine and has a mounting member disposed between the power plant and an antivibration device, wherein the mounting member has an opening abutting the power plant and an inner space continuous from the opening, a weight mass is disposed in the inner space, and the weight mass is fixed to a recess in the inner space through a first bolt. The mounting member is fastened to the antivibration device by a second bolt.