There is provided a vacuum pump mounting structure. A vacuum pump is connected to an axial end portion of a first cam shaft of an engine and is configured to be driven by the first cam shaft. The first cam shaft is disposed in a cylinder head. A cam housing rotatably supports the first cam shaft on the cylinder head. A cam housing-side boss portion and a cylinder head-side boss portion are formed in the cam housing. An engine-upper side of a body portion of the vacuum pump is fixed to the cam housing-side boss portion. An engine-lower side of the body portion of the vacuum pump is fixed to the cylinder head-side boss portion.