A fixing hole and a locating hole of an automobile's roof are aligned in a predetermined direction while a fixing part and a locator pin of an antenna unit are aligned in a slightly different direction on the condition the antenna unit is oriented in the predetermined direction. The antenna unit is placed on the roof and a male screw is engaged with a female screw fixed to the fixing part to attach a fixing attachment. By tightening the female screw, the roof is held tight between the antenna unit and the fixing attachment and thereby the antenna unit is fixed to the roof. In this case, the antenna unit is oriented in the predetermined direction with a high degree of accuracy due to clearances between the fixing hole and the fixing part and between the locating hole and the locator pin.