A base assembly comprising: a base; a gasket; and a cover, wherein the base includes a fixing part to be fixed to a vehicle body and an attaching part, an upward portion of the attaching part being attached to a monitoring assembly, the gasket is attached to the fixing part to exist between the fixing part and the vehicle body, the cover is attached at a downward portion of the fixing part and at a downward portion of the attaching part to cover the downward portion of the fixing part and the downward portion of the attaching part, and wherein among edge parts of the cover, an edge part of the cover that faces an edge part of the fixing part at the downward portion of the fixing part and faces an edge part of the gasket at a downward portion of the gasket has a vertical edge part that is provided in a vertical direction, the edge part of the gasket at the downward portion of the gasket has a horizontal edge part provided in a horizontal direction from the edge part of the fixing part at a downward portion of the base toward the vertical edge part of the cover, characterized in that a plurality of stopper parts (7F, 7B, 8F, 8B) to restrict the vertical edge part of the cover from moving with respect to the edge part of the base at the downward portion of the base by action of the horizontal edge part of the gasket are provided at the edge part of the fixing part at the downward portion of the base and at the vertical edge part of the cover, respectively, and the stopper part (7F, 7B) of the base is provided in a vertical direction, and the stopper part (8F, 8B) of the cover is provided in a horizontal direction. the base includes a fixing part to be fixed to a vehicle body and an attaching part, an upward portion of the attaching part being attached to a monitoring assembly, the gasket is attached to the fixing part to exist between the fixing part and the vehicle body, the cover is attached at a downward portion of the fixing part and at a downward portion of the attaching part to cover the downward portion of the fixing part and the downward portion of the attaching part, and wherein among edge parts of the cover, an edge part of the cover that faces an edge part of the fixing part at the downward portion of the fixing part and faces an edge part of the gasket at a downward portion of the gasket has a vertical edge part that is provided in a vertical direction, the edge part of the gasket at the downward portion of the gasket has a horizontal edge part provided in a horizontal direction from the edge part of the fixing part at a downward portion of the base toward the vertical edge part of the cover, characterized in that a plurality of stopper parts (7F, 7B, 8F, 8B) to restrict the vertical edge part of the cover from moving with respect to the edge part of the base at the downward portion of the base by action of the horizontal edge part of the gasket are provided at the edge part of the fixing part at the downward portion of the base and at the vertical edge part of the cover, respectively, and the stopper part (7F, 7B) of the base is provided in a vertical direction, and the stopper part (8F, 8B) of the cover is provided in a horizontal direction.