In a sensor mounting structure, a sensor is mounted in an outer portion of the floor panel in a width of the vehicle, and a step garnish, extending in a length of the vehicle, is attached above the outer portion so as to cover the sensor. The step garnish may have a rib extending in the length of the vehicle; and the sensor may be positioned inside the rib in the width of the vehicle. Preferably, the step garnish has a plurality of ribs extending in the length of the vehicle; each rib has clip portions for attaching the step garnish to the floor panel; and the sensor is positioned between two of the ribs. Typically, the rib protrudes downward from a lower face of the step garnish. The floor panel may have a concave portion; and the sensor may be attached to a bottom of the concave portion.